Thank you for starting this. I recall it was several years ago a similar thread ran....then disappeared.
I've been hankering to scratch build an MOW car or two based on photos in the MOW book for my favorite RR, ATSF. This thread will be my incentive to move it up on the 'bucket list'.
One reason I believe that MOW trains are seldom modeled is that they're usually simply parked....awaiting disaster or repairs...hardly a scheduled run! OTOH, if you belonged to a group who celebrated 'operation' as a part of an evening together, scheduling an MOW train to handle a wreck or maintenance would certainly be a way to spice up the night....and take a beer-break for those in charge of the fast-mail, priority passenger, perishables freight, etc., etc..
Otherwise, for those of us who favor simply running trains, an MOW set of cars is usually at home parked on a siding. And that's not all bad, mind you! It certainly can make for an interesting display of creative reconstruction of old cars. The paint/lettering is often ho-hum enough to make a project easy, too.