This information is somewhat dated but I think its essentially the same today:
How Diesel Locomotives Work
Driving a Locomotive
"You don't just hop in the cab, turn the key and drive away in a diesel locomotive. Starting a train is a little more complicated than starting your car.
The engineer climbs an 8-foot (2.4-m) ladder and enters a corridor behind the cab. He or she engages a knife switch (like the ones in old Frankenstein movies) that connects the batteries to the starter circuit. Then the engineer flips about a hundred switches on a circuit-breaker panel, providing power to everything from the lights to the fuel pump."
Much much more... 3 pages if you want to read the whole article
http://science.howstuffworks.com/transport/engines-equipment/diesel-locomotive10.htm