+1. I think it's significant that most of the respondents to this thread are really eager to travel by train - so long as the bike remains a bike - but do not even consider the train if that involves disabling and bagging the bike. This applies even to those (like me) who are happy to spend a great deal of of time and trouble dismantling and packing their bike so carefully that it'll (usually) survive airport baggage handling unscathed!
Q: Why this apparent double standard? Why is a lot of very careful dismantling okay to go on a plane, but any dismantling at all (less careful because one will handle the bike oneself) NOT okay by train?
A1: Because a 'plane is so much quicker, making all the time one needs for dismantling and reassembly. And...
A2: More importantly: the 'plane probably goes direct to a good starting point for one's touring holiday. And even if there is a change, one's bike and other luggage will be handled by the airport.
Travel by train, on the other hand, usually involves changes of train. A bagged bike plus a number of panniers adds up to an awfully difficult burden to shift between platforms, given that porters are a thing of the past and the trolleys meant to replace them are often just as absent. Add the fact that for most Brits any continental rail journey involves not just a change of platforms but a change of Paris termini (last time I was at Gare du Nord the trolleys - so I was told - had all been stolen!) and those difficulties become virtual impossiblilities for all but the young and strong of back and arm!
If the bike remains a bike however, it ceases to be an additional awkward burden, but remains one's own perfect personal luggage trolley, purpose-made for trundling panniers etc from platform to platform. And should one's connection leave from a different station, there is no need to interpret a foreign metro system, one may simply get on and ride across the city centre!
Another thing, A3 if you like. Although one may not need to be so careful how one bags a bike if handling it oneself, the size limits on bagged bikes imposed by train operators are often much smaller than those of most airlines. On Spanish long-distance services for example, one is permitted three items of hand luggage, the largest of which cannot exceed 85x55x35cm and the total of which cannot add up to more than 25kg in weight or 290cm in size (calculated by summing the three dimensions of every item). A bagged bicycle is allowed to be one of those items, subject only to the restriction that its three dimensions do not add up to more than 180cm. I wrote 'only', but to save you doing the maths, I can tell you that one would have to dismantle a regular touring bicycle almost completely, probably filling all of the three bag quota with bicycle parts before even thinking about one's panniers!