I think there is one thing to remember: even if a puppy has not experienced everything they will ever encounter in their lives, they can still get used to things at an older age. I have three rescue dogs, none of them were ever on public transport before we got them. Now, two of them (the third one never had to) are absolutely perfect on trains, trams etc. and settle down without problems... in fact, Dylan wants to get onto every train that goes past when waiting at the station.
Yes, it is important to socialise a pup properly, but that doesn't mean it has to see and hear absolutely everything it will ever see and hear as an adult; but it means that the puppy is taught to getting used to new situations so it won't have any problems getting used to new situations as an adult dog that he has never encountered before as a puppy. In other words, if a dog learns to cope in new and perhaps stressful situations (by socialising it as much as possible at a young age), then they will do just that as adults!
I fully expect to be able to take our two older dogs onto any kind of public transport now, eventhough they might never have been on it before, I fully expect them to adjust to new scenarios quickly and trust me, even if they have never seen it before.
It is an individual dog thing... our second lurcher comes from a very similar background as our older lurcher, but he is much less flexible, is still worried about new situations, takes much more time getting used to things... whereas our older lurcher is comfortable and confident in almost any situation that life throws at him now!
So, yes, go and socialise your pups as much as possible (hey, so will I in a few weeks when we bring ours home) and take it wherever you can; but don't panic if you are not able to do one thing or the other... chances are good that an otherwise well socialised dog will be able to adapt to almost any new situation.