I was booked on a regular NEC regional train from New Haven to NYP (connected from Springfield, much cheaper than going from Providence which is the same distance... anyway...)

The regional rail train from PVD broke down just short of New Haven. I talked to a ticket agent minutes before an Acela was coming through -- she just wrote the Acela train number on our tickets and said "run for it, get on the Acela." We did. The conductor didn't bat an eye. Note: we didn't have reissued tickets, nothing official. I could have written the train number on the stub and had the same effect. Note also, several other people waiting for our broken train did NOT get on Acela because they didn't ask. So: they were good in IRROPS, but you've got to ask for help....