[David Morgan's lexicon](http://www.wyomingcatholiccollege.com/faculty-pages/patrick-owens/lexicon/adumbratio/index.aspx) (warning: big file) suggests [*citatus, -a, -um*](http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Dcito2) for "express" in this context. His suggestions for "express train" are:

> tramen* citatum, hamaxostichus* citatus

(The * indicates that it is a modern word found after AD 1400. I won't comment on which is better, since that's not really the OP's question.)

Here are some sample uses:

> *citato equo* = "at full gallop"

> *citatum agmen* = "rapidly marching column"

> *citate* (adv.) = "speedily"