My experiences riding French trains during the summer of 1994 was that there are quite a number of trains where your bike can travel conveniently with you for free. Certainly not the TGV's but there are many trains with convenient baggage cars or where you can take your bike as "hand luggage." So if you are willing to take a train within a few hours either way of a particular time, it should not be a problem. Overall, I was very pleased with the convenience of the French trains.
Unfortunately, it can be difficult to work out which trains have baggage cars or permit bikes as hand luggage. If you ask three different people, you are likely to get at least two different answers. The little light-blue colored train schedules are pretty easy to read. If there is a little picture of a bike in the subscript of the train description, you can take your bike for free. "Velo gratuiut" it will say somewhere if there happens to be a key for that particular schedule. Unfortunately, the correspondence between those little blue schedules and the actual trains is vague. In general, there are more trains than appear on the schedule and the scheduled times have shifted from the printed ones by +/- 15 minutes, it seems. In any case, the most authoritive info comes from the big white books they have at the train stations at the sales or information desks. But often the people reading them are not too familiar with the bike stuff so there is some chance they will make a mistake. Also, sometimes the big white book info about bikes will disagree with the little blue schedule info.
If you do find a train on which your bike is allowed, there seem to be two possible ways to travel with it. If there is a baggage car, you can put your bike in there, fully loaded, locked if you want, and forget about it until you get where you are going. You may want to sit near the baggage car or at least get near it before your stop since it may be hard to get your bike off quickly if it is a short stop. If there is no baggage car, you will have to find room for it in one of the areas at the end of a car. I found that it helped to stand it upright so that it didn't block anything and that since I was nervous about people taking things off it, I either stood or sat near enough to see it. Note: my XO2 is not a big bike, but since it has front lowrider and rear racks and dirt-drop bars it is pretty bulky. Certainly with all the panniers it is quite bulky.
If you take a train which does not allow "velo gratuit" then you are supposed to pay FF155 and then they put your bike in a box and send it on a different train. It is guaranteed to arrive within 5 days. Clearly it is worth the trouble to wait for a train you can ride your bike with.
Spain was less convenient. They also have the "your bike will take a different train and arrive within 2 days" policy like France does. Bikes are permitted on some trains, though. So this is what I worked out: the "Intercity" trains bikes are not permitted, but there is plenty of room to put your bike on the shelf at the end of the car if you take the wheels off and compactify it a bit. I did this and nobody complained, although I was worried that they would. The "Talgo" trains are another story. These are another intercity train and there is (evidently) no space for bikes except in the walkways between cars where they are pretty much in the way. They may boot you off or not let you on the train in the first place on these "Talgo" trains. They didn't let me on with my bike, so I didn't get much of a chance to look and see if there was enough space for a compactified bike. I ended up riding my bike for that leg, sigh, it was hot (37 deg.) There may be some variation from conductor to conductor, so it may have just been that I happened to get a real hard-nosed one. I'm not sure and my Spanish is quite weak so I don't think that helped much.
Overall, though, I found the trains very convenient.