Backhoe - OK, Trackhoe - ????
The backhoe thing I've seen done. Actually the BN using this arrangement to unload ties this last summer between Claremore, OK and points toward Afton, OK. And having a couple years experience operating backhoes, I don't see a problem with an experienced operator doing this. I haven't myself, but never had the opportunity to try either.
The backhoe in the pics is specially equiped for this task - guide on the rear outriggers, behind the front wheels, and also on the front bucket. With the front bucket being 8' foot wide, and as long as the digging bucket is kept pretty close to straight behind the tractor, the process would be fairly stable. TWO big concerns have already been mentioned, brakes on hopper and hydralic line failure.
Still don't quite get why they would ship this kind of material in a hopper if they were going to have to unload it this way. First thought was shipper ordered wrong type of car, and the company at the other end had to resort to this to get their material unloaded. But looking at the bucket on the rear of the backhoe, it is oversized considerably. Someone has spent some money on that bucket, more than for a one-time job. And that bucket is not suited to digging holes or trenches - no teeth and would make the tractor unstable at full reach with dirt, rock, etc. Even with the oversized bucket on the back, it seems time intensive to use this method. Any one know how many cubic feet or yards a hopper like this would hold??
As for the trackhoe in the other pic, I've operated those too. Only two ways I can think to get it up there: 1)ramp 2)crane. Is this the only hopper you see the trackhoe on? If so, is one end of the hopper hinged? If their using a ramp, or ever a seires of "steps", a 45 degree angle would be safe, because you can either extend the boom uphill as a counter balance, or downhill as a stabalizer on the ground. Either would work depending on the CG of the machine. That machine could probably negotiate "steps" 3' high X just longer than half the length of the tracks. In this case, why not buy a backhoe - their a lot more adaptable machines and travel (drive) much faster.
I've run out of ideas on this one, any others out there....