Not easily but they tend to be Markets where the express operator either sees more potential than mainline does not has other business reasons to operate them (such as a maintenance base). Timing may not be as tightly coordinated with mainline banks.

IIRC the last time I read a UAX operator contract (which was a while ago so things may have changed) for at risk flying the airline also has to pay for fuel which is covered/provided by the UA for "fee for departure" (non at-risk) flights