Will I need to use compensation or springing?
(updated 02-06-16)
In a few cases, such as long wheelbase 4-wheel wagons or coaches, compensation may be of benefit. Worsley Works offers a simple etched inside bearing compensating frame, which may be useful, and the Society offers etched compensating W-irons. Otherwise, for wagons, generally no.
For bogie coaches the well known MJT bogie kits, stocked by the Society, are recommended. They include a neat simple compensation arrangement, and give very smooth running. Uncompensated bogies can be obtained from 3SMR.
Locomotives generally don't need compensation. However, some form of flexible chassis arrangement may improve running and offers opportunities for improved current collection. But you don't need to go down this route unless you want to.