I just saw some pix showing the overhead caternary system 35 years ago, was told the trucks going down (and using dynamic braking) were feeding to the system, and the trucks going up were drawing from it.
There may well have been systems like that back in the day but I can honestly say I never saw once personally. As I said above the main drawback would be the capital cost of setting up a second trolley line for trucks descending into the pit would effectively double the capital cost of the system. This, coupled with the inefficiency of the energy transfer from DC back to AC in order to return the power generated by the descending trucks retarding to the power grid, kind of makes the idea a non-starter for financial reasons.
Operating cost-wise trolley works really well in countries that have an abundance of cheap power, usually those countries with a well-devloped hydro-electric system. In other words electricity has to be much cheaper than diesel.
Another thing that the proponents of trolley-assist conveniently forget are the grading requirements of the haul road under the trolley line. It has to be almost perfectly smooth and graded to within an inch of its life so that the overhead catenary is no less than Y and no more than X metres above the road surface. It can't have any soft spots, dips or humps because those can potentially cause the truck pantograph to separate from the catenary and if that happens the truck loses the trolley-assist. Not good when you are climbing a 10% ramp fully loaded with the diesel engine at idling speed.......!! From an operational standpoint the requirements to maintain the road under the trolley line is a full-fledged nightmare.
The other downside of trolley-assist is that once it has been installed the mine is pretty well stuck with that road alignment for all haul traffic coming out of the pit until the system is eventually decommissioned, because moving the installation would probably cost more than installing a new system in another location in the mine. Not ideal if the pit in your mine is dynamic and constantly expanding, or changes in profile majorly year on year.
To give you some idea on the cost, here a some numbers sucked from a 2019 report regarding the capital cost of a trolley-assist system for an existing diesel-electric truck fleet that would require a sub-station every 850 metres (935 yards) of trolley line.
Trolley Line installation (structures, cabling, etc) per 850m - $2.3m
Substation (ea) - $2.8m
Pantograph & controls/truck - $350,000
So based on those numbers to construct a 2.55km trolley line (3 units) for a 30-truck fleet would set the mine back $16.3m in infrastructure and $10.5m to modify their truck fleet - $26.8m in total.