I saw someone arrested on the platform at Nailsea and Backwell for assaulting a guard on the train. I didn't see the actual assault, but heard him being threatened and saw him react in a manner that was consistent with being assaulted.
I still have the notes I wrote on my phone at the time while my memory was fresh as I gave my details to the police (BTP I presume) to come and take a statement from me later. The file on my computer is dated 7 October 2007 and it's possible that that was the date it happened, although it might have been before then (I got that phone in early 2006).
I've cleaned up the spelling and put some notes in square brackets, but otherwise this is verbatim:
On 1810 train between Bristol TM and Exeter. 2-car Class 158 DMU
Between Parson St and Nailsea and Backwell the guard started to check tickets in last carriage starting from back of train.
A passenger in the 2nd or 3rd row from that end started shouting at the guard. Said that he had been told by a member of rail staff to travel to WSM [Weston-super-Mare]. Threatened to punch the guard in the face (not exact words but this was clear meaning). The guard handed back tickets (two bits of card) to passenger(s) on opposite (right in direction of travel) side of train without turning away from disruptive passenger. Guard then went directly to the rear cab of train and shut door. I presume that he then spoke to driver as train stopped quickly afterwards (came to a stop opposite signal B19 [a northbound signal]).
While the train was stopped the disruptive passenger at first stayed in his seat and ranted to those nearby (i was about 4 rows back on same side of train) about having an appointment with the DHSS (I think he said in WSM), about being charged £200 by British Rail / First Great Western (used interchangably) for being homeless, about BR/FGW calling the police to him at WSM and making him miss appointment.
Then [he] went up and knocked on the door of rear cab. When no answer a male passenger traveling in the vestibule area tried to calm him down (based on actions, didn't hear words). Then returned to seat and continued rant. Did say that the "ticket lady at Weston" [i.e. Weston-super-Mare] should have known about him as she watched him get on the train after talking to her for 20 minutes.
At some point during this a large man I presume to be the driver came through the train and used his key to enter the rear cab and talk to the guard. Eventually the train got underway again after driver returned to front of train. We arrived at Nailsea and Backwell. After the doors were not opened for a couple of minutes the driver again came through train to talk to guard which he did briefly. Driver then opened doors. Disruptive passenger remained on train until the police were walking along the northbound platform towards the footbridge [the only access to the station is via the northbound platform] when he left the train (whether he saw them or heard other passengers comment on their arrival I don't know). When he saw the police officers descending the stairs to the SB platform he went towards them and made a stance a bit like the rugby Hakka pose, as this was seen through the train window I did not hear anything.
One police officer stayed with him while the other went to talk to the guard and driver who were by now on the platform (I didn't notice when they got there).
My suspicion is that the disruptive passenger was at least slightly drunk. Another passenger (who did not give his details to the police) commented to this effect as well after the train departed Nailsea and Backwell.
I was about the second last passenger to board at Bristol TM so I did not see the passenger before then. My attention was not drawn to him before he started shouting.
This statement was written starting about 5 mins after leaving Nailsea and Backwell station and continued and concluded in my car in the car park adjacent to the southbound platform at Yatton. Concluded at 1924 (arrived Yatton circa 1840).
When the police came to take a statement at my house a couple of hours later (I was living in Cheddar at the time) they commented that I was the only person who was prepared to give their details to the police.

They were I recall particularly impressed that I'd noticed the signal number for the location, but slightly frustrated by my not being very accurate in my estimation of distances (I have always been bad at putting estimations into numbers).
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A couple of years after that I was on a HST to Paddington that came to an emergency stop somewhere east of Reading followed by a tannoy call for any police or BTP officers to make their way to the buffet car. I was in the adjacent standard class carriage and two passengers make their way rather purposefully towards the buffet about a minute apart. Presumably off-duty police travelling in different carriages.
I didn't see or hear anything else at the time, but when I alighted at Paddington I overhead staff members talking and it seems that a man had attempted to steal cash from the buffet and had been arrested.