How can freight trains be made lighter without reducing the actual freight?

The week long closure of the line through Pewsey this week for track renewal got me thinking this question, as this route has lots of heavy aggregates' trains along it. Heavier trains of course increase track wear, so what potential is there to reduce the mass of a freight train while still retaining the same volume/mass of product?

This would obviously mean using lighter locos and wagons. The Class 99 is ~17t lighter than a 66 (113t vs 130t), while all electric locos can be under 100t. Is there any potential in making wagons lighter (i.e. new wagons with lighter material), particularly large aggregate ones?

Make aggregate wagons have 3 wheel bogies, increase wagon weight, decreases payload but in theory redices track wear as each wheel on the wagon imparts less force onto the rail.

Perhaps because wagons tend, in my impression, come from traditional steel fabricators more modern materials such as alumminium are not being used where perhaps they could be.

Track access charges depend (among other things) on the assessed track damage caused by a particular vehicle type, so reducing track forces is one factor in the economics of freight operation. Some bogie designs exert less track force for the same axle load. The original low track force bogie had inside bearings, which probably also reduced the weight of the bogie itself as frames were smaller. This formed the basis for the bogies on the 220 and 172 but never really caught on for wagons.

Reducing the payload per wagon would mean more wagons to handle the same load, which may actually result in more track wear along with all the other cost and capacity issues it would create. So it seems pretty universal to max out the payload within the permitted axle load, and take any reductions in empty weight as extra payload rather than less axle load.

I seem to recall that Foster Yeoman (as was) bought or leased some aluminium-bodied aggregate wagons but they just didn’t last long before structural weaknesses manifested themselves.

Surely the first thing to consider is payload weight v empty weight, obviously keeping within axle load limits.

The problem is if you adopt a lighter design it is likely to have a shorter life, and vehicle cost will become an issue if using exotic materials, so the whole thing become a juggling exercise to find the 'sweet spot'. I suspect that current designs have probably evolved to be close to this optimum. If one of the operating costs changes dramatically then the sweet spot may move.

I also wonder how other aspects affect overall efficency, how much drag does wind resistance add at tyical freight speeds, then friction losses.

I suspect if there were any quick wins they would already have been taken, so you would be iinto the small gains area, and the costs of implementing them would make them unattractive.