The GP7-GP9 discussion got me thinking has anyone ever run into trouble with the EPA before because of the emission standards I know historic equipment is usually exempt but what happened when retrofit say a 645 or 710 to a GP7 or GP9 to get it functionesl instead of useing its origenal prime mover does it then count as a rebuld and need to meet emission standards?

I would think if the replacement engine is not a current manufacture it may still be exempt, you are not operating the engine on a daily basis?

I'm going to call attention to 40 CFR Part 92, and 40 CFR Part 1033. I won't quote them directly here, as it's easy enough to look up this information on line. The most recent copy I have is 2009, so, it may have changed slightly.

Basically, locomotives built after 1973 are required to meet EPA emissions standards at their next overhaul or remanufacture, which is basically if all power assemblies are replaced within a 5 year time span. At that time, the engine must be brought up to the maximum emission standard that is available for that model. This became effectiv in I believe 2008.

I believe, companies that are listed as 'Small Business' can petition to become exempt. To be classified as 'Small Business', I believe there is a maximum number of employees, I think 1,000 or so. However, I think this includes the parent company as well, if the RR operation is a subsidiary.

Here at Grand Canyon Railway, we had 2 of our 3 F-40's remanufactured to Tier 0 and Tier 0+ compliance. Our parent company Xanterra has around 8,000 employees, so we don't fit into the Small Business Category.

I have never had an EPA official come onto the property to verify our compliance.

A handy guideline for the date of manufacture is that EMD shifted almost all production to "Dash Two" specs--modular electrical/electronic components--after 1972. If you have to ask whether it might be an SD40 or SD40-2, there you go. There are exception, of course, such as the MP15 and the F40PH.
GE locos: U-boat production spanned both sides of the cut-off date; anything a "Dash Seven" or later definitely qualifies.
Only a few MLW locos pass the cut-off date, and save for M636's they're mostly "odd duck" models anyway.