Title says it all. That's what happened to me yesterday morning.
Staff communication was quite amusing:
Owing to a trespass incident between Bristol Temple Meads and Filton Abbey Wood all lines are blocked.
2K12 being delayed at Stapleton Road whilst the driver makes an emergency GSM-R▸ call due to passengers on the line trying to retrieve a dog that is on the line.
All on stop at Stapleton Road due to passengers on the line trying to retrieve a dog.
Update 1 at 09:33: Dog has been 'secured' and has now got loose again.
Update 2 at 09:35: Trains still on caution. Dog is secured after 2 attempts by the rope from the emergency cupboard - dog was caught the second time after a passenger donated their lunch to the platform to entice it back up off of the line.
Update 3 at 09:45: Normal working resumed, everyone clear of the line. Dog in the care of BTP▸ - congestion building in the Bristol area now as a result.
Novel use of emergency rope and a passenger's lunch there.

Slightly more seriously is the issue of
pax▸ trespassing to rescue the dog. One hopes they got a stern talking to from BTP.