Conducting Power on a Scratch Built Chassis

Several reasons. One, because I have two spare "Thomas types" and only one unused Flat type (which is missing the positive terminal, the switch and its cover). The one from my custom Ivor is currently slated for a permanent conversion project. Two, the relative simplicity of the "Thomas type" chassis makes them look easier to imitate, and they can still provide power to the drive unit after being reinserted. Three, soldering would surely make the drive unit harder to remove when transferring to another model (and somewhat defeat the purpose of what I have in mind). Fourth, I've never actually soldered anything and don't have the proper environment to do so (even if I would've had the tools and safety measures for it). Not to mention, the side effects of lead and other fumes would be twice as dangerous to me, and it interests me how not all motorised items conduct their power in the "obvious" way.