FRA recommendations for
Locomotive Cabs
Engineer Seats
4.4.1 Operational Issues
Engineers spend long hours in close contact with locomotive seats and are understandably greatly concerned with their comfort. Most cab characteristics that the engineer may perceive as unpleasant are somewhat transitory (e.g., too hot, too noisy); however, the seat is a constant. Seat acceptance is influenced by intrinsic seat characteristics and by factors that the operator associates with the seated position. For example, Roach and Rockwell (1980) found that a too high cab temperature was the single factor that most influenced seat discomfort. This is probably due to the heat and perspiration levels experienced at the seat contact area. Other non-seat factors that can affect seat acceptance include: leg room, knee room, availability of footrests, clearance from sidewall, vibration levels, ease of entry/exit, clearance when swiveling, visibility, and reach-to control distance. In fact, Roach and Rockwell found that seat characteristics accounted for only 46% of the variability in the seat ratings for their survey. Therefore, any seat assessment has to be done in context with cab workstation design. The seat type installed in locomotives depends on the property which buys (or owns, for replacements) the locomotive. Some are specified by the railroad management and others are specified by labor agreements. The long service life of locomotives means that older models with more austere seating are still in service. Thus, seats in service range from relatively advanced designs to a round seat pad that resembles a stool. Similarly, the seat adjustments available and the nature of the adjustment mechanism vary by railroad and locomotive type. Operating a conventional locomotive is a relatively fixed position task that restricts the engineer’s ability to move around or stretch at will. The modern console style cab puts even more limits on the engineer's ability to change position. The older control stand allowed standing operation and required twisting and leaning to perform some operations. This means that the engineer could move more, which prevents postural fatigue. The console allows (and requires) less postural variation. Those systems which still have a deadman pedal impose extra constant leg effort. The justification for concern about seat comfort is provided by Roach and Rockwell's extensive survey. Nearly 90% of participating engineers had a criticism about their seats. About 80% reported discomfort after a typical run. Their complaints included high and low back pain, hip and buttock pain, neck pain, and leg aches. The long-term implications of these pains and the underlying physical changes that cause them needs consideration. Riley et al. (1991) obtained similar pain ratings for the SD-60M seat which indicates that the advanced design has not addressed basic seat issues.
4.4.2 Human Factors Considerations
In most rail operations, the engineer must remain seated for extended periods of time. Sitting changes the weight-bearing surfaces, restricts spinal movement, flattens or distorts the normal curves, and immobilizes the pelvis. Seats must compensate for these changes. Weight has to be spread evenly to avoid pressure points and the muscular and skeletal interactions of the spine have to be addressed to avoid stresses. The seat must also permit the worker to perform tasks from that position, so its relationship to the rest of the workplace is important. Seat design has many aspects that come into play to satisfy the support and position requirements of the seated body. Roach and Rockwell (1980) investigated these in their survey with prototype seats and derived a specification that incorporated anthropometric needs and situational preferences which were tailored to locomotive conditions. Their resulting guideline has theoretical and empirical support. Seats also have social status implications. The size, design, and materials have connotations of status which affect the perception of the seat and its user. A seat should meet or exceed a person's perception of what is appropriate for the situation. A seat that appears to be poorly designed or worn conveys a sense that the person occupying it or the job being performed is not important. The reverse is also true. There is no perfect seat or universal definition of seat comfort. Roach and Rockwell tested four prototypes that used good human factors design practices. While they had better ratings than the seats in use, they drew criticism on many characteristics. Riley et al. (1991) mention that the SD-60M seat incorporated many Roach and Rockwell recommendations. Engineers generally rated the seat better than others, but far from perfect. Thus, no matter how good a design standard may be, there will always be room for improvement and criticism. Aspects like cushion softness are hard to measure meaningfully and subject to personal preference rather than objective standards.
Seated Posture The human spine is intended to bear weight in a vertical direction. In proper alignment, the vertebrae transfer weight as a compressive force. Discs at joints between vertebrae cushion the vertical transfer. Sitting with hips too far forward and/or a slumped back disrupts alignment which creates uneven pressure on the affected discs, shifts the center of gravity, and adds a shearing component to the force being borne. Disc structure is not designed to resist sheering forces. The muscles and ligaments have to compensate for this which adds a fatigue component. With the accumulation of fatigue, the potential for acute and chronic injury grows because adaptive potential of the tired muscles and strained ligaments is lost. The seated position, and more so the slumped position, limits spinal motion which provides the motive force for spinal fluids. These fluids carry nutrients and provide lubrication to the discs and other structures. Fluids stagnate over long periods of sitting and the discs suffer deficits. Serber (1990) quotes work by Nachenson and Elfstrom (1964) which calculates a stress of 495 pounds at affected discs when the seated torso leans forward at 40 degrees. This is near the rupture point and requires muscular compensation to avoid damage. These calculations are for a static seat and do not consider the vibrations and jerks present in the locomotive seat which make spinal stresses more dynamic. Riley et al. (1991) observed that engineers lean forward with a slight twist to the left when operating controls and sit back in the seat when monitoring movement. This leaning may be the source of the stress that causes the acute pains reported by engineers and a potential source of more chronic problems. Low back pain was the most common complaint (40.6%) in the Roach and Rockwell study. Serber (1990) discusses three seating designs that allow pelvic motion to compensate for the lumbar distortions of leaning forward. The most common is known as the continuous balance seat. The seat pan travels in a curved track that allows it to tilt the way an unconstrained pelvis would to maintain proper spinal alignment. It creates a rotation centered at about the fourth lumbar vertebra (just below the beltline), the natural seated hinge point. While this design may correct one remaining seating problem (restricted pelvis), it is not known if it can be made strong enough to be reliable in the locomotive environment and if the dynamic motion will be accepted by engineers. Placement of operating controls on chair arms is another alternative that may keep the engineer in the position assumed during monitoring times, but this may be too radical a change for engineers to accept.
Seat Adjustments allow the seat to accommodate people of different sizes in an otherwise fixed workspace, suit personal preferences, and permit changes of position to relieve fatigue. Potential seat adjustments include: height, seat back angle, seat pan tilt, fore-aft position, swivel, armrest height, and lumbar support.
Presence of seat adjustments is not enough. The adjustment mechanisms have to be easy to use and reliable. Roach and Rockwell found that 42.1% of their survey group had trouble working the adjustments on the standard seats. Riley et al. (1991) provide a recent insight on this matter with their comment that the seats on the property they studied required two people to change seat height. This was due to the 65-pound effort to lift and support the seat in its new position while a pin is pulled and replaced in the support sleeve. Reliability of adjustment mechanisms is possibly a worse problem. A seat set towards one extreme that cannot be reset could pose real problems for an engineer who requires the other extreme. Failure of standard seat adjustment mechanisms in the 40% to 50% range (e.g., 47.3% slide, 46.4% back tilt) were cited by Roach and Rockwell. Adjustments that are too hard to work, or broken, reduce their use and negate their value. Another failure problem area is mechanical wear that introduces looseness and adds unwanted motion. Wilde and Stinson (1978) describe a seat swivel design that wears and allows the seat to wobble. The wobble probably has more of an adverse impact than the benefit the swivel feature adds. The harshness of the locomotive environment makes adjustment failure modes more critical than for mechanisms designed for a more benign environment like an office.
Alternative Designs:
The introduction of the computer, with its video screen and keyboard, to the office has sparked renewed interest in seating. New designs have been created to address problems and many of these differ from the usual upright design (e.g., forward leaning). While many of the same problems exist in the locomotive, there are different problems too. The vibration and jerk levels are very different from the office, the requirement to remain in a fixed seated posture is much longer, the need to anchor the seat limits adjustment options, and engineers need to lean and twist while seated. So, these new seat designs may bring some benefits, but create new problems for the engineer that the office worker may not experience. More analysis and testing is needed to determine if the engineer would benefit from radical seat designs. Another aspect to consider is the potential willingness of engineers to accept an unconventional seat. The need for alternative seat designs may also be important if napping is allowed in the locomotive cab. The FRA is considering the use of napping as a method for minimizing fatigue from the long hours of service and the irregular schedules that locomotive crews follow. The seat design needed to accommodate both comfortable working and napping positions may be very different from a design that accommodates only work related activities.
4.4.3 Recommendations
Seat Design The following design guidelines were developed by the Roach and Rockwell study (1980). Since there is no source of anthropometric data for railroad workers, they used 1978 NASA data for the 95th percentile male and the 50th percentile woman as the upper and lower size limits. These size ramifications were combined with analysis of engineer comments on characteristics of the different seats (standard and prototype) to devise a tailored set of characteristics. They do differ in some areas from some generic seat design guidelines that focus on the office environment.
Seat Characteristic Recommendation
Features and Adjustments Folding armrests, variable back tilt, sloping seat pan, fore-aft adjustment, variable height, swivel
Seat Pan Size 16- to 18-inch effective length for any position of the seat back Seat Pan Width 17 inches minimum at back, 20 inches minimum at front
Seat Pan Slope 1 to 3 degrees from horizontal, front edge higher
Cushion Thickness 3 inches minimum for pan and back
Back Height 21 to 25 inches
Back Width 16 inches minimum at hips, 21 inches minimum at shoulders
Armrest Height 7 to 8 inches from top of uncompressed seat to top of armrest
Armrest Width 4 inches minimum
Armrest Length 13 inches minimum
Armrest Padding 1/2 inch minimum inside and top
General Armrest Adjustment to lower elbow end to tilt 115 degrees from horizontal, armrests parallel to seat pan and 19 to 22 inches between inside edges
Seat Covering should not cause sliding or be easily torn or cracked, must permit breathing and water vapor exchange
Fore-aft Adjustment Minimum of 4 inches fore and aft of center
Swivel At least 180-degree rotation from forward to rear facing, rotation towards center of cab
Seat Height No more than 16 inches at lowest position and at least 19 inches at top position (measured at top of front edge), adjustment steps no larger than 1 inch
Seat Back Tilt From 95 to 115 degrees from vertical in steps no larger than 5 degrees
These recommendations are a good start, but do omit some other features that seating literature mentions as beneficial. Adjustable lumbar support that extends across to the pelvic bone crests is widely recommended. This provides support and corrects posture. Moderate contouring of the seat cushion for the buttocks and the seat back for spinal curves evens pressure and provides support. It can also be a subtle deterrent to slouching because body contours will not match the seat contours in an improper posture. Lateral support on seat back or a curved seat back to supplement side sway support reduces abdominal muscle effort. A continuous balance seatpan, armrest controls, or other means to relieve the lumbar stress that occurs when bending forward will address a large residual seating problem. Additional backward motion of the seat or other adjustment that makes room to permit standing operation would add to possible operating position options lost in the shift to consoles.
Adjustments The ideal seat adjustment mechanism is strong, easy to use, reliable, and wear resistant. The harshness of the locomotive operations may make office type mechanism designs unsuitable. A survey of current designs for ease of use, reliability, and wear resistance should be done by the seat purchaser to identify suitability of current designs. Efforts are required to develop criteria to evaluate new designs, and determine where further design work is most needed.
Seat Environment The workspace that the seat is placed in has considerable impact on perception of the seat. Non-seat characteristics can have a direct or indirect impact on the seated position or use of the seat. These need to be considered along with the seat characteristics to determine seating comfort. Non-seat factors that need to be considered include: leg room, knee room, availability of footrests, clearance from sidewall, vibration levels, ease of entry/exit, clearance when swiveling, visibility, and reach-to-control distance. The use of a deadman pedal as a vigilance device also has a bad impact on the seated position. Leg and knee room and footrests have comfort and health implications. Little leg and/or knee room forces immobility; the resulting discomfort can be endured for a short time, but not for long periods typical of an engineer's shift. Health aspects come into play from the blood pooling that can occur from the lack of muscular movement. Cramps are a common result with the potential to develop phlebitis which can lead to clot formation. These clots can lodge in the leg veins and cause thrombosis or travel to the brain (stroke) or lungs (pulmonary embolism). Roach and Rockwell found that 41.8% of older engineers (60 and above) reported leg aches compared to 23.7% of younger engineers. This is the only complaint that was reported more frequently than by the younger group and is consistent with the loss of circulatory efficiency that occurs with age.