Which is  easier to transport material long distances?  Universal conduits or rail carts? For example bringing coal and iron or copper and tin together to make steel and bronze.

Conduits are easier to use overall, so I would recommend them for something like an alloy smelter early in the game. You should be able to find resource deposits for an alloy smelter not too far apart from each other.

Rail carts would work well for delivering coal and iron to multiple alloy smelters. The cart could fill with coal and then drive to a series of alloy smelters, dropping off coal for each one which would be much more efficient than searching for a coal and iron deposit for each alloy smelter.

Rail carts are mobile storage containers, so they are good for consolidating things. They can go around the world, collecting wires, gears etc. and bring them to a central location where auto crafting machines can build solar panels, extractors and other machines. 

Once you are producing all the basic components such as wire, plates and gears of all types, it is a good time to set up a rail cart.