Any detector would do, I would suggest an Ace 400 because:
* You won't have to wade through menus and spend hours watching youtube tutorials to use it, and
* You'll have a very nice detector once the train thing is done
As I opined earlier, I think its very possible a target this large might overload circuitry, so I'd start out with mid to low sensitivity. Essential to learn the pinpoint function, very simple just a continuous press of a button. Practice on something visible, say a quarter, and see how the sound alerts then cuts off when waving the coil. In this way, you can define the boundaries of a metal object underground. Of course a quarter you will move the coil inches for results, for an engine would be many feet, so you are going into areas which not too many have gone before.
Also, if you get an alert, raise the coil half a fool while swinging, if it still alerts raise it more - If it is still alerting 1ft+ in the air, you can be fairly certain that you have found a large metal object, next will be to define it using pinpoint as described above, after that would come the digging. When you have defined an edge/corner, best to mark it with something, rocks should do since I'm sure you don't want to carry marking flags with you. I have found sewage traps like this, pretty easy. You could very well end up with a denied, rock outlined profile of the object, and you should be able to determine if it fits the profile of what you are looking for.
Lots of variables though, one would be if there are railroad tracks buried there too, could throw you off. But a railroad track by itself would be simple to define, you wouldn't have to go very far at all width wise before the signal cuts, so if you have something screaming in length and dying in width, you'd know that is a track.
I'd look for one used at a nice discount, craigslist/facebook is often loaded with them.