Elecrail has had the greaser set for nearly a decade, and other cars including loco L1162 for even longer (with the exception of 80M and 222M which were acquired later, which are probably also banned from running a train since they do not have the SPOT modifications; they were centre motors when they were retired, the only way they could get around this is by putting them in a 6 car set behind 7M/99M or whatever Metro calls them now, once Metro has had enough of them sitting around wasting space).
As for the red rattlers, I'm surprised that they aren't even allowed to drag them around dead behind a loco. I'm pretty sure they would still look the part if they were sandwiched between K153 and K190 on the suburban shuttles (I wish they were actually all red though without the white, just a personal thing) - of course, they might have to "borrow" some overhead power for the lights. I suggest the steam locos because they are already used on the shuttles, and the fact they have dual couplings which are quite capable of towing screw coupled stock (good luck using push-pull T classes).
Harris 794M/797M: Unroadworthy condition
Swing Door 107M/137M: Waiting to be let out of Newport
Tait cars: Waiting to be let out of Newport
Hitachi 80M/222M: Uncertain if they are allowed to actually lead a train in revenue (e.g. passenger) service
L1162: Waiting to be let out of Newport
Other L class locos: Spare parts for L1162
E class locos: Unroadworthy condition
Parcel vans: Do they even have any? If so, refer to Tait cars
MTH101/103/104: Not owned by Elecrail
4D: Found in Coles and Woolies in the cat food aisle (look for the Australian Made logo, as most cat food comes from Thailand, thus will not contain any 4D parts)
The only option for Harris "T" cars, if H cars cannot be sourced, would probably be some blue S/Z cars, or maybe the K cars (at least the windows would look somewhat similar), although the Harris set would need auto couplers on the #2 end replacing the drawbar (no big loss, considering there are no unmodified Harris trailers left anyway).