I used the smallest tungsten cubes made for pinewood derby cars, and was able to fill the cab with 6,( I think, it's been a while), and another 4 on either side of the cab on top of the frame. Oddly, upthread I asked about slow speed improvement when you remove the decoder(I use DC), and I have done it myself since then, and it was 100% worth doing if you use DC.
Also, welcome to TRW. Here is a pic of my 44 tonner, small world, eh?

For various reasons I’m invested in not going DC, but I am considering a smaller decoder. Needing to sort headlights is annoying tho