If you're going to do away with the traction tyres and fit plain wheels there a few simple mods you need to do to improve traction to an acceptable level. If you have the older Hornby or Dapol chassis, you need to get at the spring underneath the trailing wheelset, and cut it in half and then stretch it just a little. This will still keep the axle on the track and keep the loco stable, but it means that most of the loco's weight will be on the drivers. My 2004 vintage Hornby one will easily cope with three Mk1s or a mixed bag of 10 four wheeled wagons. Its a bit more tricky with the new square axled version. You can carefully slot our the trailing axle slot, but if you go too far the loco will rock on its center axle. Even if you do get it right, changes in gradient will cause problems unless they are very gentle. Bit like the first issue of the Oxford Adams radial tanks.