Is it feasible, taking into account their weight?
Yes, but for all the reasons folks have outlined, it's not for the feint of heart.
As someone one said, 'If you can meet with triumph and disaster and treat those two impostors just the same', you'll be fine.
As far as 'weight' is concerned, be sure you're capable of hefting the bikes up and into the carriage (and down and out on arrival), and perhaps lifting the front end to hang from a wall-mounted hook. Then of course, depending on the facilities at the station(s) you will be using, you
may need to lug the bikes up and down stairs. If there are two of you, that might make things easier, but ... see next comments.
Is pre-booking possible ...
Yes, and as others have said, necessary on some services. Booking presents its own challenges of course, which increase in proportion to the complexity of your journey. If just a single train between you and your destination, it's not too bad. When it involves multiple connecting services, it can become more complicated.
To book a seat on a particular service is relatively easy because there are (usually) many seats. Some trains however, may only have a handful or even just two cycle spaces, so booking on a particular train may be harder, especially if you need two (or more!) bike spaces. If you then need to book spaces on one train then another connecting service, it's easy to see how tricky it can become. In essence, book as early as possible (usually up to 3 months in advance) and if you can be flexible about your timings, that will work in your favour.
Where to book? Some online booking services don't allow you to book bikes (e.g. thetrainline.com), whereas others do (e.g. redspottedhanky.com, and some train operators sites). If a multi-stage journey, before you part with cash for your first stage, make sure you can book the bikes on subsequent stages.
Alternatively, again in plenty of time, head for a station with a ticketing office. Don't try this in a short lunch break, if you need to catch a train/bus, or if your car is on a meter; it can take some time. The cashier has to go through all the steps you would to find trains with adequate bike spaces and, from what I can see from our side of the counter, their computer system is no more efficient than what we have when making online bookings. Also it very much depends on their skill/patience/familiarity with their booking system ... and whether there's a queue of other folks waiting behind you!
Personally, I prefer to do my bookings online so I can choose the services which suit me. For example, I might be prepared to wait an hour at a station for a connecting service if I can get a bite to eat, or at least sit in comfort out of the weather. A booking clerk at a station won't know, or probably care, what variables you might be willing to tolerate.
... are the racks big enough?
As Knitty said, it very much depends on the service(s) you use. Some local services are roll on, roll off and are dead easy. Some local services and often intercity services are not, as I found out when touring with my Elephant Bike, even after removing panniers:
One further ting to bear in mind is that the space(s) you notionally booked may not be available when boarding the train. Sometimes it's another cyclist without (or occasionally even with!) a booking and if they're nowhere around to discuss the matter, you might be stymied. At other times, the bike areas get stuffed with luggage, prams and even passengers if the service is busy. It then comes down to your confidence in ... negotiating access.
One-way van hire is the other possibility
If there are two of you and even if you have discounting railcards, the cost of rail travel on some routes can be extortionate. Advance fares can provide a cheaper option, but these tickets are usually for specific services so if one train in a connection is late, getting on an alternate service can be problematic, or expensive. However, this can even be the case when you've paid the premium for an 'Anytime' ticket. One way van hire can be expensive, but still competitive when faced with the cost of train fares for two.
As a solo traveller seeking the most economic and manageable means of getting to the start and end of a tour, I'm prepared to be flexible in my travel arrangements and adaptable when the snags arise. I've found that although booking become more complex when there are two of you, sharing the burden of negotiating the difficulties when travelling makes the journey less fraught. If I was travelling as a group of more than two, I'd definitely be looking for an alternative to the train.
I should also add that if you approach railway staff in the right way, they can be incredibly helpful in helping you with your journey if things look like going awry.
Let us know how you get on.
