Thank you for the (many and varied) responses - you've given me a number of things to think about (and started a debate it seems)! Since I wrote my earlier message in a hurry, please let me clarify a few things.
First, my wife asked, and was given, permission to buy a ticket for her journey on board the train, before she boarded. So as mentioned above there can surely be no question of a penalty fare - it is completely misleading to suggest that the guard was lenient in not issuing one.
Secondly, my wife and I between us probably make around 100-200 similar journeys a year. Since the trains are every 10 minutes, for around 20% of these journeys there is not time to buy a ticket before boarding the train. On all such occasions we go to the guard and ask if we can buy a ticket on board the train (and it's not just us - it seems commonplace at the stations on this line). As an estimate, around 95% of the time the guard says yes and sells a ticket (with railcard discount for my wife - so it is possible to do this on their machine). Occasionally they say no, in which case we buy a ticket from the machine/ticket office and wait for the next train. Only once before, and now today, has she been told she can buy on board and then refused a railcard discount (I wasn't there either time).
Third, isn't the advice always given if you don't have time to buy a ticket beforehand that you should ask permission to buy on board the train? See, e.g.
http://www.passengerfocus.org.uk/fa...-train-if-the-ticket-office-queue-is-too-long
which states
"If you decide to board a train without a ticket as the queue at the ticket office was too long, in these circumstances it is advisable to get permission from station staff and/or seek out the conductor and explain the situation.
If you do get on the train without having bought a ticket and there were facilities to pay, or you did not get permission from station staff to get on without a ticket, you may be prosecuted for fare evasion, charged the full fare for the journey or issued with a penalty fare."
Surely the latter paragraph implies, by any reasonable interpretation, that if you *do* get permission to get on without a ticket then these penalties (including being charged the full fare) don't apply? My wife was given permission to buy on board (that is, to get on without a ticket), so surely according to this should have access to the full range of tickets even though she didn't specify the railcard? In any case, I agree with the posters who have suggested that in this context, 'can I buy a ticket on board' should naturally be interpreted as meaning 'from the full range of tickets'.
Fourth, she wasn't sold an undiscounted ticket - it was an off peak day return, just with no railcard discount. I don't know what logic the guard used to decide off peak was allowed but railcards not - any ideas?
I appreciate that my original post must have made me appear like someone who wants to complain about any and every perceived sleight - I hope this isn't the case! It's a very small sum of money in question (around £1), certainly not worth the time and effort of these posts, let alone following up - but I will be writing to London Midland about it for the following reasons.
1) First, as above, we commonly ask permission to buy on board, and I would like to be confident of where we stand, especially since the same situation on another train could be very expensive.
2) Second, as above, it appears lots of people ask to buy on board like we do, so this is an important question that affects many people.
3) Finally, and most importantly, my wife was really upset by the incident. She feels that the guard was 'shouting rudely' at her in front of many other passengers. Whilst I wasn't there, I know my wife well(!), and she would go out of her way to avoid creating (or being part of) a scene. From her account this guard was, to quote an earlier poster 'a knob', who behaved in an completely unacceptable manner. Almost all the London Midland guards (from whom we regularly buy tickets) seem very friendly people, and this guard is letting down all his colleagues, and should not be allowed to continue in his position unless he changes his attitude. But how are London Midland to know if no-one complains?
The reason I asked on here was I wanted to know the actual legal position for this situation. In particular, I thought I remember seeing a document which explicitly stated that *if you are given permission* to board a train without a ticket, then you can buy from the full range of tickets on board, and I hoped someone would provide this - since no-one has, I guess I must have imagined it.
p.s. someone asked for the exact conversation prior to boarding - it was apparently:
'I haven't had time to buy a ticket, is it ok if I buy it on board?'
'Yes - just get on and come to the door at the back [indicating the door to the guard's compartment]'
Thanks again for the replies, infomation and suggestions.
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Another source - not official, but showing at least that this is a big area of uncertainty:
From:
http://conversation.which.co.uk/transport-travel/travelling-without-a-train-ticket-too-complicated/
"That aside, you must get a ticket no matter how long the queue at the ticket office is, according to the conditions in the rulebook. And yet watchdog Passenger Focus contradicts this by advising that if it’s more than five minutes’ wait at peak times and three minutes’ off-peak you can try hopping on board without a ticket – but get staff permission first.
If there isn’t a ticket machine or office or – more likely – the ticket machine is broken, then you can get on without a ticket. In all these circumstances you’re only supposed to pay the fare you would have paid if you’d bought the ticket just before getting on the train."
That is, if you have staff permission to board without a ticket (which my wife did), then you should only pay the fare you would have paid at the ticket office (i.e. railcards should be allowed). Correct or not?