To add to Brett's explanation. FRA requirements, as outlined in
49 CFR Part 230 require that when a locomotive is initially built or returned to service, so as to
operate on a FRA regulated railroad, the boiler would need to be surveyed, evaluated and verified that it would meet the required safety factor. These calculations are recorded on a Form 4 and filed with FRA. However, having a Form 4 does not equal FRA compliance. As Brett explained, all the other requirements of 49 CFR Part 230, including, minimum condition of the running/driving gear, brake system, etc. along with documentation and verification of all FRA inspections is also required. This includes the recording of service days, periodic filing of inspection reports and FRA inspections.
A locomotive can be rebuilt, surveyed and have a Form 4, but that does not make it FRA compliant. All other items also have to be compliant and specific periodic inspections performed and documented. The later was not done with the 346 and 491, as it wasn't necessary. The 346 would almost be at the end of it's 15 years (as per FRA), so would have to start with another boiler survey (Form 4) in addition to all the mechanical issues that would need to be addressed. The 491, has a Form 4, but probably would have trouble meeting all the other FRA requirements.
Mike Ramsey