The use of passes started in the early days of train travel.
Passes were issued to employees and families and those people the railroad wanted to "treat".
Passes were often issued for a set time and limited to certain routes and dates.
Passes were printed on heavy card stock and were about the size of credit cards.
Passes were numbered and had the word "PASS" on the face.

Model railroaders adopted the creation of the passes for their model railroads.
Today, many model railroaders have created a "PASS" for their empires.
Trading and collecting passes has become a  recognized part of the hobby.

Use of the computer and e-mail, gave way to the "electronic pass" (e-pass)
and the trading of those passes.

The National Model Railroad Association currently has exchange programs
for both printed and electronic passes.

 National Model Railroad Association, Pass Exchange section at
 http://www.dickg.com/~dickg/nmra/sigs/PassExchange.html
 
The Railroad Paint Shop at http://paintshop.railfan.net/e-pass.html

The above information was heavily borrowed from: http://www.kilchis-river.com/