Well, in the 1830's and 40's, obviously the rail lines being built were mostly the big intercity rail links. Liverpool-Manchester, London-Birmingham-beyond, East Coast Mainline, Great Western Railway and so on. Thousands of tons of freight and hundreds of passengers per month were probably the order of the day for that. A backwater region like mine (16000 odd souls and a few industries here and there, with at best some 80 tons of cargo per month per train) would be connected much later generally, with simle tank engines or perhaps even a shunting engine.
That said, Simutrans normally of course has less traffic potential than real life, so perhaps something can be said for lowering the running costs.
In my experience, a locomotive that has a running cost of no more than 1.50 should be fairly profitable with a train of some 75 tons. A locomotive with less than 1.00 should be profitable pulling 50t and for one of 2.50 or more you need at least 100t.
Also, in my experience, one needs between 4 and 6 profitable trains to offset the maintenance cost of a not-too-expensive rail line in this era.
I hope that gives you an indication.