I believe the old rule was 36 months, but was often extended to 48 months for locos not in continuous service. I'm not sure if there was a maximum limit for out of service credit (which must be taken in blocks of 30 days or more.)

I think the 15 year limit was set to be sure that at some point the boiler would be reinspected, especially where long term storage might have led to corrosion issues. There are several engines out there that might only average 10 days of operation per year - so they would be able to operate 147 years before hitting 1472 operating days! (Think of the 700, 4449, 3751, etc.)

It is expensive - $100k to $750k+ depending on the size of loco, the work needed and if any of it can be done by qualified volunteers. Many organizations spend their time fund raising to pay for multi-year rebuild projects, but then fail to consider the business aspects of operating the locomotive and having the money available to pay for the next inspection.

Assuming you did get all 1472 days of use (over 4 to 15 years) - if the inspection/rebuild costs $300k then you need to set aside $200 per operating day just to cover the next one. If you only average 10 days per year, then you need to set aside $2,000 per day - probably double that for a big loco. This doesn't include normal running repairs, fuel, labor or the tack, train, advertising and insurance.

NOTHING specific to the railroad is cheap - figure it will cost four times as much as a common industrial item.

Rumor has it that the FRA is investigating a days used inspection schedule for diesels that only operate a couple days a week. Under the current rules, a locomotive that operates once every three weeks has the same inspection requirements as one operating 7/24. In addition to the expense of the 92 day inspections, you also get a bill for $5,000-$10,000 every time you have to change the air brake equipment. (Yearly for the old stuff, 3 years on most of the newer.)