I used to run a couple doing traction only on scrap for one of the big metal firms. We ran it alongside a few on general haulage and also a couple of motors of bulk waste work, so I got a sense of how the money compared.
I’d say the rates worked out maybe a bit better mainly because we were doing all local work around the M25 on the scrap, so we could get a few loads in on a good day. But then you have to take off it that your servicing costs are higher, you’ll shred tyres, and even when the loads are light the fuel is pretty rubbish because of the aerodynamics of the trailer.
Also not all drivers want to do that “pushing on a bit” type of work. The handy drivers are ones that don’t mind a bit of dirt, will still look after the wagon and keep it clean, and will give it a go when you say “reckon you can get another one in by 4pm” or whatever.
In case you’re serious about it, we learned there’s a few ways to get from loss to profit in it. For example, if you do traction only some sites will let you skip the queue and just drop the trailers on the yard, if one of your later guys (or a mate) will run it over the weighbridge. Also, not all jobs need tipping so some you can run with a regular unit. And as someone else said, it’s Mon-Fri, 7am-4pm type work, so I mixed it around with nights work for a parcel network on the same unit.
Hope it helps.