Wasn't all of this explained in an earlier thread?
The MKII ac EMUs (classes 317-322) have a common traction design, i.e. transformer/rectifier set to produce a motor supply of up to 750VDC (nominal). The motors are conventional DC types so can be controlled by varying the input voltage level. The normal method is to have a tapped transformer and switch between those taps to vary the (low) ac voltage fed to the rectifiers. this gives a step changing DC feed for the motors. Tap changing on transformers means that there is no need for further equipment to vary the voltage applied to the motors. The arrangement will only work for ac though.
The class 319s are different from the other MKIII ac EMUs because they are required to run on 750VDC 3rd rail or 25kV ac. It was decided to incorporate electro-rheostatic braking in the 319s with a system that would work on both types of supply. This required a common motor supply rail for both power supplies. Instead of using resistors, solid state DC-DC converters were chosen, allowing integrated wheelspin and braking to be included. The space required for this early electronics hardware was not available under the floors of the pantograph/motor car so it had to be accommodated under the trailer.
The coaches are organised thus:
DMSO (or DTCO) - minor auxiliaries including compressor
MSO - pantograph, transformer & motor bogies
TSO - DC-DC traction controller
DMSO - minor auxiliaries including compressor
so as you can see, the driving end cars are the only ones with enough room to mount a motor/generator/fuel/exhaust system
For the record, the other MKIII ac EMUs pack all of the major hardware into the MSO as the DC-DC unit is not required, so they can run as 3-car units if required.