My son called me to ask how dogs can travel by train. As I've never had to do it I couldn't tell him. Does anyone know? Do they have to be crated or can they go beside the seats?

Dogs must be kept on a lead at all times unless contained in a basket.
Dogs without leads, cats, birds and small animals must be carried in an enclosed basket, cage or pet carrier. It must be rigid and not open (to prevent escape) and the animal able to stand and lie down in comfort.
Animals and containers must not occupy seats, otherwise a charge will be made.
Animals are not allowed in restaurant cars except for blind persons' guide dogs or deaf persons' assistance dogs, which may be allowed into the restaurant car at the Steward's discretion. However, every effort is made to serve meals at blind or deaf persons' seats within the normal seated accommodation.
Blind persons' guide dogs or deaf persons' assistance dogs are allowed on Sleeper services provided the blind or deaf person and dog are booked into single-cabin accommodation. The guide or assistance dog is conveyed free and no cleaning charge applied.
ScotRail allows dogs to accompany able-bodied passengers in Sleeper services subject to a charge for a heavy duty clean, and the booking must be either Standard Class with two people travelling together, otherwise a Solo supplement is payable for exclusive use of a twin-berth cabin.
Booking for all dogs travelling in Sleepers should be made at least 48 hours in advance.
Byelaw 16 allows the train company to refuse carriage or entry to any animal. If a customer objects to the presence of an animal, the owner must move it to another part of the train or put it in a suitable container in the guard's van/luggage van where available.
When dogs are carried in the guard's van/luggage van (other than those in a container) they must be secured with collar and chain and properly muzzled.

This is for the National Rail service.

http://www.england.worldweb.com/Transportation/Trains/

A link for the different train companies and their information pages.

http://www.england.worldweb.com/Transportation/Trains/

A link for the Tube - couldn't find any info for pets.

That's great info Mr. Spock. Thank you, I'll pass it on.

I have taken my dog on the train many times with no problems.

I reserve a seat with a table & she just goes to sleep.

Terry, my wife has just returned to London from the St. Patrick's Day Show in Ireland. She took two dogs with her by train and ferry and had no problems at all. Not only that, but it worked out much much cheaper than going by car.

and they travel for free.

we took our dog to scotland on the train (6hr journey) she sat under the seat and slept (hehe its a great way to get all 4 seats to youself saying there is a dog under the table)