Much would depend upon what supposed purpose the express reefer was serving. If it had highly perishable goods onboard, it would be essentially limited to the consist of some high speed, long distance passenger train.
In the case of more common, longer-lived produce, if a standard reefer wasn't available, it could have been employed and placed on a secondary train, or fast freight...and if it happened to hold something like say potatoes, it might simply be included on any old freight train.
In short, express reefers saw a wide varitety of service situations beyond what they were specifically designed for, especially if more common reefers happened to be in short supply at the moment.
So...it is really up to the operator to assign a supposed cargo suitable to the situation that places the reefer in a particular consist, passenger or freight.
CNJ999