Honestly, there is nothing significant about the bell to identify it any more specifically. It's about the right size for a locomotive bell--that's about it. There just isn't enough information to go beyond that.
It's like if I handed you a spark plug and asked if you could tell me what make and model of automobile it came off of. Our only information would be that it's a spark plug--and it might not have even come off a car.
Any markings *might* be under the wood, but I doubt any are there. If you had the yoke and cradle for the bell, that might have been useful in determining if it came off a locomotive (a rounded base, for instance, indicating it might have been used on a round boiler, or the design might have indicated a builder). Even then, specifically identifying what particular train engine it came from would also be impossible. Some bells did have engine numbers stamped in them. Your's doesn't seem to have any markings.
As it is, you're just going to have to answer the questions as best as you can, and if you don't know the answer, you're just going to have to tell people that. You're getting plenty of reasonable bids--don't sweat it.