Should hostels target more locals than overseas visitors? Encourage train travel over flights? (apparently, planes generate 10 times the emissions of trains per passenger)
Or is it something that hostels shouldn't be concerned with?
To the last sentence, yes.
Hostels exist to provide certain types of accommodation to people who arrive in their town or area. In general, they should cater for whoever wants to come through their door. The most they can do is to promote themselves over other types of accom eg hotels, to get a greater proportion of those people than they would otherwise.
The promotion of the town or area should be done by the local tourist organisation. A hostel should be a member of that organisation, but the task is wider than one hostel can do on its own.
Regarding train, car or air travel, it's up to the people doing the travelling. Major factors are price, convenience with luggage, whether they run when you need them, etc. If trains cost much more than car and more than air, few people will use trains whatever the climate benefit. Some hostels offer a discount if you arrive by public transport, but so often public transport is not a realistic option :( .