DEN has two basic security areas - one in the main terminal and the other at the end of the passenger bridge leading to the A concourse. The main security area in the terminal has two sides, both of which lead to the same train station beneath the security area. You can use either side. Both sides have an Elite/FC line that is for use by those with status on any airline. It is not UA specific.

If you are flying an airline that leaves out of the the A concourse (CO, F9, LH, BA and Ted) I would take the pedistrian bridge rather than bother with the trains. It is an easy walk and the security lines are typically shorter than those in the main terminal. It too, has an elite line although most of the times I go through the regular line is so short it makes no difference really.

In general, the security lines in DEN move rather quickly. Unless you will be passing through on a Monday morning or Friday afternoon the wait times shouold not be bad. If you can use the elite/FC lines the wait will be minimal, especially though the A concourse line.