Express doesn't use the United airline code or callsign.
They used Republic 3541 for BDL-EWR:
http://flightaware.com/live/flight/R...000Z/KBDL/KEWR
EWR-SDF operated as Republic 633:
http://flightaware.com/live/flight/R...015Z/KEWR/KSDF
Originally Posted by
lincolnjkc
If I recall correctly, there are only a handful (maybe 5?) of "valid" suffixes -- (T)ango being popular, but I think (K)ilo and (R)omeo are also in that pool.
I think I was told it has to do with ease of reading/differentiation on radar screens (not being confused with a number, like say (I)ndia could be)
You've either been informed incorrectly or recall incorrectly. At some point in the last 20 years UA has used all 26 letters of the alphabet for suffixes.