Well, that photo above is more or less what it should look like, but at the moment it is just a body kit, as supplied by Nonneminstre Models.
However, my P4 train set needs trains! The kit is designed to fit a motor bogie, but that would look very obvious below footplate level. I did get as far as starting a scratchbuilt chassis some time ago.......
......but like most things I ever do with locos, I wasn't totally happy with it. One problem was that the design relied on a long chain of spur gears to get the drive to both axles. It looked more like a cheap clock that a loco chassis when it was assembled - not at all elegant. There was no room to hide the motor, so it would have been vaguely visible inside the cab. The other problem was that the prototype is built on wagon principles, with a frame at buffer level and W irons for the wheels. This means that there is lots of space underneath and very little is hidden. To do a good job, I really needed to keep all but the final drive around buffer height or above - and not in the cab, if at all possible.
On the plus side (I must try to think positively!) the compensation arrangement, with a transverse beam on one side of the chassis, worked well. The chassis seemed to ride more elegantly than a wagon I tried with the more usual one-axle compensated W-irons. The final gear deliberately looked a lot like the gear on the prototype's chain drive too. That's two features worth considering for 'Mark Two'.
So what to do? Well, firstly I bought a Ruston 48DS kit, hoping for inspiration. I wasn't completely happy with that either (see other topic) - aren't I hard to please?!

There are a few other issues I need to resolve before I finish this project, like couplings, loco livery etc.
I do have ideas, but they will have to wait until the next instalment.