Active transit tunnels are especially alluring sites to those who ride through them every day, but abandoned tunnels once used by passenger or freight trains hold a charm of their own.

What constitutes a transit tunnel?

A tunnel which was originally built for the purpose of facilitating traffic. This includes automobile tunnels, freight train tunnels, subway tunnels, and even covered waterways. People who like exploring transit tunnels also usually like exploring the stations between the tunnels.

How much risk is there of getting caught?

A fair bit, if you're using tunnels that are still in use. Exercise extreme caution while entering and exiting the tunnels, paying attention not only to the eyes of other passengers but also to surveillance cameras. These cameras are installed to monitor for violence, not to watch for tunnel runners, but it's still wise to stay out of their line of vision.
       Toronto Transit Commission (TTC) Security officers are sworn as 'special constables' of the Metro Toronto Police Service. The plainclothes officers have powers of a police officer and can enforce the Criminal Code, the Controlled Drugs and Substances Act, the Liquor Licence Act and Section 17 of the Mental Health Act within the TTC system. So they could arrest you.
       According to an article in the Toronto Sun, the fine for being caught in Toronto's subway tunnels is a mere $63.75. This seems low, and the TTC's by-laws suggest a more realistic sum of $500.

How much risk is there of getting injured?

There is a very good chance of getting killed when exploring still-in-use subway tunnels, though that doesn't make it any less fun. The trains themselves are probably the greatest threat, as they travel at unpredictable intervals and at very high speeds, and will not be able to stop for you if you get in their way.
       Although many people do survive accidental contact with the electrified rail(s), it's better if you avoid it altogether. Certainly don't imagine that rubber soles will protect you. The electrified rails are sometimes referred to as the "third rail", but this nickname is less appropriate in systems like Montreal's Metro or London's Underground, where there are multiple electrified rails.
       Other hazards to watch out for include uninsulated wires and, in some cities, rats.

Will I get attacked by "mole people"?

No promises, but you're probably safe. The phenomenon of people living in transit tunnels only really exists in New York City (and possibly Moscow), and the numbers of people living in tunnels under New York City have been steadily declining since the problem first attracted media attention in the mid-1980s. This lifestyle trend hasn't caught on elsewhere, though it's a popular subject of urban legends.

What supplies should I bring when exploring transit tunnels?

At least one flashlight and several beverages per person. Moist towelettes are also essential, since you'll get pretty dirty. In some situations, it may be helpful to toss on a safety vest and hardhat and go for the disguise angle.

What preparations are necessary before entering the tunnels?

Knowing is at least half the battle. Track down whatever transit system maps you can find to locate as many potential opportunities and threats as possible. Ask people who've been in the tunnels before for advice. Be aware of any construction in the areas you'll be visiting. Have a clear idea of how often the trains are passing through the tunnels — though don't by any means count on the trains sticking to a regular pattern.

Where can I see people infiltrating transit tunnels on screen?

Bone Collector, The (1999) - After becoming a quadriplegic from an accident in a train tunnel, Lincoln Rhyme (Denzel Washington) advises a lady (Angelina Jolie) on further excursions into transit tunnels.

Blues Brothers, The (1980) - The escape from the performance hall goes through Chicago's coal tunnels.

Dark Days (2000) - A documentary profile of a semi-society of houseless people living in the vast unused areas of a train tunnel in NYC.

Ghostbusters II (1989) - Ray (Dan Aykroyd) is lowered into a long-abandoned subway system; later Egon (Harold Ramis), Winston (Ernie Hudson) and Ray explore another part of those abandoned subway lines to find another route to the river.

Godzilla (1998) - Mutated iguana Godzilla takes up residence in the subway tunnels under Manhattan.

Jackal, The (1997) - Bad-guy terrorist The Jackal (Bruce Willis) flees into D.C.'s subway tunnels, but is pursued and thwarted by good-guy terrorist Declan Mulqueen (Richard Gere).

Johnny Mnemonic (1995) - Johnny (Keanu Reeves) and Jane (Dina Meyer) wander through subway tunnels and an abandoned subway station (Toronto's Lower Bay).

Matrix Revolutions (2003) - Neo (Keanu Reeves) tries tunnel running, only it's a mobieus tunnel and he ends up back in the same station.

Mimic (1997) - Mutated cockroaches capture scientist Susan Tyler (Mira Sorvino) and take her to the lowest levels of the New York subway system (Toronto's Lower Bay).

Money Train, The (1996) - Transit security guards Charlie (Woody Harrelson) and John (Wesley Snipes) do some infiltrating to rob a New York subway train filled with money (filmed in LA's Bullring Yard, a former Southern Pacific Railroad hub which holds a 3/4 mile "tunnel" with two tracks and a variety of station types).

Relic, The (1997) - Alien bad thing inhabits tunnels underneath a museum (filmed in Chicago's abandoned freight tunnels).

Speed (1994) - After the nonsense with the bus, the misfortune of LAPD SWAT member Jack Traven (Keanu Reeves) and girl Annie (Sandra Bullock) continues when their subway car speeds so fast it goes off the tracks.

Superman (1978) - Lex Luthor (Gene Hackman) inhabits a lair inside the subway tunnels of Metropolis.

Toronto's abandoned Lower Bay subway station has also served as the set for commercials for (in chronological order): Oh Henry chocolate bars, Compaq computers, Carl's Jr. Restaurant, and most recently, Pall Mall cigarettes.

Where can I read more?

The Mole People (1993) - Jennifer Toth's popular (though suspect) account of the lives of thousands of houseless people inhabiting the gas, sewer and subway tunnels under New York City. Urban explorers in New York know this book to contain inaccuracies.

Infiltration 5 contains articles on Toronto's subway and LRT tunnels, Infiltration 12 contains the history and a tour of the Chicago Tunnel Company tunnels, and Infiltration 13 features adventures in the under construction Sheppard Subway line in Toronto.