19:45 King's Cross -> King's Lynn

[cmovcc]

I was on this train, 2nd coach from back; in the tunnel before Potters Bar there was a huge crash/bang noise (which sounded like it was under the train), which continued for about 10 seconds inside the tunnel, and then another 20 odd seconds outside the tunnel (with the overhead lines twanging madly), before someone pulled the cord and the train stopped past Potters Bar.

the driver went to investigate, and said that the train had pulled up a lot of ballast causing damage, and that the train would have to be taken out of service.

we then reversed up the down fast, and were detrained at Potters Bar.

after we were detrained he didn't find any more info, so presumably didn't see anything reversing up the line.

what on earth could cause that sort of noise under the train?

 

[yorkie]

It appears to have been a train fault of some sort.

The following Grand Central & East Coast services to Bradford & Newcastle were delayed by around 11 minutes, they probably had to use the slow lines to get round your train, but arrived on time. Later trains were not delayed at all. So I don't think it was anything to do with the track or other infrastructure.

 

[High Dyke]

AFAIK. A member of railway staff travelling on the 17:55 Peterborough - Kings Cross service reported something hitting the underside of the train/object dragging on the track between Hitchin and Stevenage. Driver examined the unit, couldn't see anything amiss but did report ballast damage to the underside of the train. Units: 365514 / 365539

The 19:45 KGX - Kings Lynn was the same two units.

 

[cmovcc]

detached sander pipe apparently

I especially like the bit where the train went via Peterborough, but nice to get a reasonable explanation for once

 

[causton]

I am presuming the Peterborough reference is because PBO = Peterborough, PBR = Potters Bar, people usually see the latter and think the former, so if the OP said Potters Bar someone probably shortened it to PBR and someone went 'oh, Peterborough!'