While we are chatting, will it be rigid or sprung or compensated? The 4' Shunter was an easy decision. It MUST be suspended some way, with only four wheels on the track. Classic Sharman suspension was the obvious answer. The driven axle rigid to the frames and the leading axle tilting on a fulcrum. It never fails. Unless the pickusps corrode, obviously. They have not over a year now. None other of mine have. I must have had some corrosive gas in the vicinity. My flux is a dilution of phosphoric acid. It is not terribly corrosive. So I have no explanation. The three other kits are sprung with, again, Slater's components. I find Slater's hornguides easy to work with, assuming that I have a set of jig axles to set them up. I don't trust rigid to run well. Are yours digital or analog?