I made a VERY simple 5" trolley, it's under piles of stuff at the moment, but at some point I'll did it out and write it up.
Basically a frame from 19mm square steel tube, welded up with a cross-bar as a foot rest. Wheels run in ball races fitted to pivoted aluminium blocks with rubber blocks as suspension (a la Mini) which works really well. I made a connector blocks that can take a chain for 5 and left or right hand 3 1/2 use.
For ground level track the seat (padded leather on plywood) sits on a hollow wooden box. For elevated track, leave out the spacer box, and fit two 'Z' shaped extensions that provide lower, wider footrests.
It has a disk brake but not fitted as I don't need it with my puny loco 
Neil
Edited By Neil Wyatt on 29/01/2020 10:50:27