How to identify MTH model numbers

Two suggestions:

1. Search the MTH site with 2-8-0 as the search term. That will give you pictures of all MTH 2-8-0's. I would suggest using the advanced search function and search separately for Rail King and Premier. Most likely it's Rail King, maybe from a starter set. 

2. Post a photo of it here. Somebody may recognize it. 

With MTH's advanced search you can select 'Steam Locomotive', 'Road Name' and 'Cab Number'. That might produce something or get you close? Knowing if it is Railking or Premier you can select that as part of the search as well. As already mentioned, they have a good number of starter sets with 2-8-0 steam for power.