Yes, trains in the United States and many in Great Britain do feature a type of security camera inside the cabs and externally as well. Also called onboard surveillance cameras, there is a multitude of reasons why each type of train features them on its exterior and interior.
For long-distance trains including all TGV, Intercités & Intercités de Nuit overnight trains, reservation is compulsory, but there are usually seats available even on the day of travel and you can buy a ticket immediately before the train departs. But there are much cheaper fares if you pre-book.
Book Early, But Not Too EarlyBut don't try to to book too far in advance, 90 days is the sweet spot. Definitely get reservations for TGV, Thalys and Eurostar, especially if you're choosing popular travel dates. For TGV, Thalys, Eurostar, and some Intercité trains, reservations are mandatory.
Onboard. In most circumstances, you must purchase a ticket before you board a train. On most Amtrak trains, only the full, undiscounted, unrestricted fare will be available for purchase onboard the train. This is regardless of reservations made or fares previously quoted by ticket agents, Amtrak.com or elsewhere.
Paris Metro tickets and faresIn the Metro you can use 1 ticket and make any transfers between lines with only that 1 ticket so long as you're underground and in the Metro system itself. Keep it with you until you exit because some Metro or RER stations require it for you to leave.