I've mentioned this before somewhere, but just to bore you I'll repeat it... the Battle of Prestonpans is famous among us railway buffs as the first recorded battle involving a railway- the Tranent to Cockenzie Waggonway, which no doubt carried the coals to the salt pans. Cope's forces used an embankment on the railway as an emplacement for his guns and a breastwork for his infantry- quite appropriately since up to the 19th century railway embankments were usually called batteries.

If ye were waukin' I wad wait,
Tae gang tae the coals in the morning.

The "coals" probably means the line that carries the coals, and if General Cope had been on the ball, the singer would have hesitated to attack such a good defensive position. However, the Jacobites outflanked Cope and attacked from the rear, routing his inexperienced force.