I'd like to go by train from London to Dover and see the Castle and Tunnels. Then I'd like to take the train to Brighton and spend a couple hours, then back to London. Is this a feasible trip by train? Can I just ask for this route at the window in London or would I need to book something in advance?

There is no need to book any of your legs in advance. However, you'll be spending a lot of time on the train. It's 2 hours from London Charing Cross to Dover Priory. It's 2 1/2 hours from Dover Priory to Brighton with a change at either Ashford International or London Victoria. It's 1 hour from Brighton to London on either First Capital Connect or Southern. See www.nationalrail.co.uk for timetables and prices.

This would be virtually impossible to do if you wanted to spend anytime in each place as the trains along the south coast to Brighton are slow, very slow and take hours. It can be done but you'll be on the train several hours. If doing it strongly look at the LondonPlus rail pass which covers all of Southeast England, including all trains you'll be taking. It costs $70 for two days on unlimited travel in an 8-day period, with longer versions also available. Since you are not going out of London and back on the same line, in which you can get greatly reduced day return tickets, you'll have to pay top fare for each leg - go to the site TimS mentions for fares in pounds. The LondonPlus pass can also be used to go to from Gatwick Airport (but not Heathrow) so if flying in there and going on this circuitous route look at the pass versus fares. These passes are only sold in the U.S. thru RailEurope - i always recommend BETS (www.budgeteuropetravel.com) for their UK rail expertise -a RailEurope agent they'll only charge a $5 mailing fee rather than RailEurope's $15 (for orders under $200). If you're traveling with kids under 16 then they each get a free LondonPlus pass the same type as you do. 3rd thru 9th members of a group traveling together get 50% off what the first two pay - so this pass is a real boon for groups and families. The LondonPlus pass also qualifies you for a passholder fare ($75 each way) on Eurostar trains London to Brussels/Paris even on peak periods where the cheapest Eurostar ticket could be $150 - so folks taking the Eurostar could in some scenarios save money with the LondonPlus pass even if they throw the pass away!

And the LondonPlus pass can be used on any train on a walk up basis - some of the cheaper tickets are for certain trains only - walk up fares in UK can be very high so flexibility is another key with the pass. There may be some kind of Network Southeast pass available as well and sold locally but these have to be used on one train company line and UK has dozens of separate private train lines so if you taking the same train lines, like Southern Trains, then you can buy real cheap day passes but you have to know your train companies and what they serve.

Thanks for the in-depth information. And for letting me know that it's not a good idea unless I want to spend all day on a train (that's why I come to Fodors - someone, somewhere knows what I need to know - I love it here!).

I'd stick to just Dover. The castle/tunnels will easily fill 5 hours so w/ the travel time to/from it is a full day's trip.