Thingking about "bundel deals", even if there were similar restrictions to some of the cycle bans in the south east (i.e. you can't joing a train which will arrive in Londen or leave London between certain times within a certain area) it would still be fairly attractive. OK a little more confusing but given it should be easy enough to define the times of the trains that they are entitled to catch on the booking website and email confirmation, so no need to explain to the user the rules as to why not other services.
I would suggest that it would proberbly be best to limit the percived number of choices that the user is given, maybe by offering them a "choice" of up to, say, 6 services that they can use as part of the booking process (even though in reality they may be entitles to travel on more services than that). That should be enough that they could pick an early in the day time (if they decide they want ot leave early), a few around the sort of time that they think that they want to leave (without having to wait too long if they just miss a service) and maybe one or two much later (if they want to stay later).
Likewise there would proberbly be times of the year when there wouldn't be the offers, for instance, GW wouldn't allow them to be sold to go away to Cornwall for the August Bank Holiday weekend.
The other thing that would make it more attractive is if it was linked with other offers (i.e. group save) so the perception was that a family was getting a better deal than a couple. As there is an argument that by getting people intrested in travlling by train when they are young they would be more likely to do so when they grow up.
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One other way of getting more people to travel by train would be raise the age at which they have to buy tickets from 5. It proberbly wouldn't need to be by much, maybe start charging when they are 7, but again it would attract more families to travel by train and encorage children to travel by train when they are older.
It would also mean that more adults would be travelling, so proberbly wouldn't cost all that much more. If that was seen as too costly by TOC's make a child ticket cost no more than a fixed amout, say £5, as long as they are traveling with an adult. As then although the TOC would be making a loss on the child they would proberbly still be making a profit on the adult, which would hopefully fill a few more empty seats which otherwise wouldn't brought them any money anyway.