It is a silly question I know but I am relatively new at this and I was always wondering what do numbers on trains mean. specifically numbers like 40 of 1218 on old steam trains.
Welcome to the wonderful world of railfanning.
Railroads assign numbers to their locomotives for accounting purposes, and those numbers are displayed on the locomotive in order to positively identify it.
Many railroads had locomotives numbered 40. As of 2016, Nevada Northern #40, A 4-6-0 built by Baldwin in 1910, is the locomotive this number brings to mind. It was and is a very ordinary locomotive, running on a not very significant tourist traffic rail line in the approximate heart of nowhere - Ely, NV.
1218 is an entirely different breed of machine. Built in the Roanoke Shops of the Norfolk and Western Railway, she is one of 43 class A 2-6-6-4 articulated locomotives. The As were one of the most successful fast freight locomotives ever built, developing up to 6300 horsepower and fast enough to protect passenger schedules on a very busy (and very profitable) system spanning several states. The Norfolk and Western has morphed into the Norfolk Southern, and 1218 is currently displayed under cover at the Virginia Railroad Museum in Roanoke VA.
Other numbers you might watch for are Union Pacific 844 (4-8-4,) 3985 (4-6-6-4,) and 4014, the Big Boy 4-8-8-4 currently being overhauled at Cheyenne WY.
[EDIT: All of the above are steam locomotives that exist today.]
Chuck (Nevada desert tortoise)