Old railway sleepers often in reclamation yards. Oak generally and worn enough to not worry about. A good chainsaw and jobs a goodun!
He only needs them 2" thick so you would be tired sharpening the saw by the time you had cut a sleeper into planks because in my experience sleepers are full of stones & grit and will quickly ruin any cutting blade!
i checked greenheart there ( ive never even heard of it tbh ) and knowing little about the timber world all im really going off is the crush strenght which with greenheart its nearly twice what iroko is so unless im mistaken its way too hard, but if i wanted a floor to be strong then it would be the real mc coy by the looks of it.Greenheart would do nicely. But probably cheaper to use softwood and replace regularly.
if i was to put 3-5mm chequer plate on it id have to rise it at least 40 odd mm to make up the space lost with something, and there is nothing else much better to do that with that timber, it would be placed with about a 2" gap between them to take 90% of the load.Timber under what plate sorry??
id have to string a lot of box on top of the existing rails to keep the floor in any decent shape at all.box under chequer plate works just fine, you can also them make a removable pannel that slots in ( so you can just lift it out when you need ot beat up the brake cylinders and slack adjusters )
sorry, i was taking them to mean the whole floor area.id much rather pack the rails up with box than put chequre plate over wood.
just run across the top of every rail with box where the chequre is going to go, then exposed timber where the tracks / wheels go