Simon,
I think we need a little bit of information from you ….
What are your 'Railway' interests … Standard or Narrow Gauge …. GWR, SR, Welsh, Quarry etc
What do you want to do with it … Ride behind it or sit in a deck chair and watch it go round?
Where are you planning to run it? Is their any physical size/weight/cost limits?
I own 16mm (SM32) + 3.5" + 5" gauge Coal fired Loco's (Both purchased RTR and Home Made)
Dave