As a break from layout building I have been tinkering with my Bachmann 150. First up how to rewheel them to P4 (or EM for that matter)
Real class 150s have a wheel size that equates to 11mm in 4mm scale. My preferred Branchlines wheels only come in 10.5mm, 12mm and 14mm. Since wheels dont get bigger but do wear down I have used 10.5 for my conversion.
The Bachmann original uses a stepped split axle held together by the gear. I have drilled out the gear to 2mm and used a solid axle instead. This means I need to rearrange the pick ups a little bit.

You can use one side as is but on the other side you need to cut off the pick ups where they connect to the brass bearings (I have also cut off the pick ups that extend upwards but you dont need to if you are doing a simple wheel swap - I will come back to why I have done this later.
To rearrange the pick ups a simple piece of 0.3mm NS wire is soldered to the pick up to bear on the back of the wheels.

New wheels fitted - make sure you fit the insulated wheels to the side with the new pick up and on the same side of the vehicle throughout. You will need to file the pinpoints off but if a simple rewheel is all you are after you can now put it all back together and the job is done - simples!
The trailer just accepts P4 wheels as it comes so a simple swap is all you need.
However if you find (as I do) the idea of a passenger vehicle being full of chassis not a good one here is one option.

Cut the end of the chassis off as per the picture - the step in the cut is to hide as much as you can in the cab. The trailer is very free rolling and the unit only has to move itself so I decided I only need 1 powered bogie.
For the new trailing bogie I modified a MJT one as per the following pic

This means all of the new brass bogie is hidden behind the sideframes - you will need to cut the sideframes off the spare powered bogie and a little bit off the back/bottom of the air bag for the sideframes to fit. You will also need new holes for the bearings to fit into.
I mounted a piece of 60 thou plasticard across the hole in the floor at the inner end of the unit and mounted the (flat) MJT mounting plate onto a piece of 2mm plasticard to give the correct ride height - I will fit extra pick ups to this bogie when I come to DCC it.

Putting everything back together and using a sticky pad to fit the motor to the floor your chassis should look like this. I soldered wires to the pick up and did away with the PCB all together
Which means this much is now visible through the windows

HTH
Jim