Technically, and way over simplified, under the Railway Labor Act (RLA) our contract does not expire and any changes are essentially amendments, including future "new" contracts. Our current UPA has an amendable date of January 31, 2017 (Section 25-A).

If both sides agree, anything is possible at any time so all an extension really means is the official amendable date is pushed down the road. For example, the company could say "we don't want to negotiate all the sections but we will offer a X% raise if you agree to an extension (i.e. a new amendable date)"