Similar situation happened on my train this past July.

I boarded Train 174 in PHL heading for BOS, it was already running 20-30 minutes behind schedule. While waiting on the platform, there was an overnight train sitting on the other track. I was getting the vibe from snippets I was hearing that they'd been having some kind of power problems on the train. They pulled out just as we were boarding.

Sure enough, we weren't even clear of the rail yard outside the station when we came to a stop and were told that another train had broken down, and we'd be pulling up next to them and transferring the passengers over. Sure enough, it was the overnight train.

After sitting next to them for a while doing nothing, they then announced we'd be reversing back into the station and they'd transfer there. So back we (eventually) went.

Needless to say, it was a totally full train, but I didn't see that it was standing-room-only anywhere. And the process was slowed down by people from the disabled train complaing that there weren't seats together.

Lastly, for the "no good deed goes unpunished" file *our* train then started having engine problems at NHV, where we had to sit and wait for an engine replacement. It also didn't help the frustration-level when the NEC train that was scheduled for 90 minutes later than us passed us minutes after we arrived at NHV.
All-in-all we arrived in BOS over 2 hours late (because I missed both the 7:15 and 8:20 commuter trains home from there, had to wait for the 10:20).

-Rob