There are two types of senority. One is company senority that applies to benifits that are earned as a result of being with the company. Those would be vacation, and health insurance. That senority transfers from work unit to work unit.
The other type of senority is work unit senority. For example if you stay within one department (p&d, linehaul, shop, clerk)for 10 years your work unit senority is 10 years even if you have been with the company for longer. If you transfer from one department to another your work unit senority starts over at the bottom. If you return to your old work unit then you start over at the bottom on the senority roster for that work unit.
Having said all that, those fellows I referenced started in Linehaul prior to my transferring to linehaul. Therefore, they gave greater work unit senority than I. The kicker is that my company senority applies in regards to vacation choices above the above referenced fellows.