Since the passenger train evolved in the days of the steam locomotive, it stands to reason that passenger comfort accomodations would be based on existing technology. Now were not talking wood or coal stoves in wooden cars here. For the purpose of this discussion, we are a bit beyond that. Passenger comfort is mostly affected by temperature. Heating is just a matter of piping steam throughout the train, tapping the steam line in each car and heating radiators along the car's walls. A fairly simple process and done the same way until after the entrance of AMTRAK and electrical car heating. Even purely electric locomotives like the GG1 had to provide steam because of the interchangable nature of passenger cars. Cooling started out as pumping melted ice water through radiators and then blowing air through them. Later, a form of steam assisted cooling was used. With the advent of diesel locomotives, railroads had to find a way of providing steam to the passenger car systems. Some railroads used heater cars which contained steam boilers but most bought locomotives with boilers located within the carbody. Some, like the WP and the GN, used both boiler equipped locomotives and heater cars due to the extreme cold winters in which they operated. In "E" and "F" units, the boiler or boilers occupied the rearmost part of the body. In most geeps, the boiler was in the high, short hood. The very last boiler equiped locomotives, like the SDP40F's, had the boiler in the extreme end of the high hood because by then, chopped (low) noses were pretty much standard. As long as there were passenger cars in service that used steam for passenger comfort, a steam source was necessary.
A locomotive, be it steam or diesel, has a front end. The front end of most steam locomotives was the end as far as possible from the cab and when some railroads switched to diesel, they kept the bulk of the locomotive between the crew and the front end. This arrangement is known as "long hood forward" and was uncommon in the west. Most, if not all, western railroads prefered the "short hood forward" due to better visibility for the crew. The assignment of the front end to a locomotive has a number of implications but the effect on the locomotive electrical system is most dramatic. When the engineer turns on the headlight labeled "Front", he expects the correct lamp to light. When he puts the reverser handle in "forward" and opens the throttle, he expects the locomotive to move in the desired direction. Class lights, number lights, gyrolights and sanders are also affected by the "front" assignment. The effect of direction on the engineer is pretty important too. When sitting, facing the front of the locomotive, he could be reasonably comfortable for some period of time. But, locomotive controls and seats were (and are) simply not designed to be operated in reverse for more than a short time. There were many cases, particularly in passenger service, where the locomotive would have to be run in reverse for the entire return trip due to the lack of turning facilities. So, in a "dual control" locomotive, there are two sets of controls, one on each side of the cab. One set is for running long hood forward and the other is for running short hood forward. Thus, an engineer could operate the locomotive out and back in reasonable comfort, just by switching sides of the cab. Dual control does not mean that there is no "front end". On the Southern Pacific, the short hood end is the front on all geeps. That's where you will find the legally required letter "F". So, dual control is a concession to crew comfort and operating safety. And not just in passenger service. The Western Pacific bought their GP7's, GP9's and GP20's with dual controls.
Consider the case of a generic D.C. traction motor as found under the 5623. In power mode, electricity flows from the main generator, through each traction motor armature, through that traction motor's field and back to the main generator. Thus, each field is in "series" with it's armature. The coils within the field and armature form opposing magnetic fields which force the armature to rotate. Actually, the entire traction motor case tries to rotate but it is firmly fixed in place within the truck frame. Since the armature is geared to the axle, the axle turns and the locomotive moves.When going down hill or coasting, the traction motor armature just spins freely within the motor. If one were to connect the motor field directly to the main generator and connect the armature directly to a resistor grid, one could produce an interesting effect. Put a bit of electricity into the field and the traction motor becomes a generator. Since the armature is connected to a resistor, the electricity produced by the revolving armature is sent to the resistor, it heats the resistor and the resulting energy is dissapated as heat. Vary the voltage put into the traction motor field and you vary the voltage produced by the revolving armature and you vary the amount of energy the resistor must dissapate. The net result is that the armature has a hard time turning because the voltage it is producing is being shorted out by the resistor. The higher the field voltage, the more retarding effect there is on the armature. This effect is refered to as "dynamic braking" and, the 5623 does not have the capability due to the lack of resistor grids.