For 4-4-0s and 0-4-4s, I use a system introduced by Horton in the Railway Modeller of July 1964. It mimics the three-point suspension of coaches where, apart from rotation, one bogie is allowed to move in all directions but the other can only rock back and forth thus holding the vehicle vertical. In the case of a 4-4-0 (or 0-4-4) locomotive, the driving wheels are mounted in a rigid sub-chassis that is free to pivot fore and aft and the rest of the weight is transmitted to the track via a pillar onto the unconstrained bogie. Theoretically, because the 4-coupled unit is rigid, current collection via the drivers is not a good as with other systems but pickups on the bogie are possible.
Alan