IN A CLASSIC battle of the big lobbies, the coal slurry pipeline bill is inching slowly through Congress. It's the coal industry and the electric utilities versus the railroads and some of the western farmers. But amidst the scrimmaging among the teams of lawyers and public relations players, it's important not to lose sight of the large public interest in this bill.

It would provide a federal power of eminent domain for the pipelines and, without it, very few will ever be built. Almost any route must cross the tracks of a railroad that hauls coal, and the railroads, who can recognize a competitor when they see one, won't voluntarily let them through.

But competition can help hold down the costs of coal delivered to power plants, and that in turn will help hold down consumers' electricity rates. Congressmen who vote against this legislation are turning down a rare opportunity to do something actually useful about people's utility bills.

The railroads, incidentally, are not defenseless in this competition. One of the first coal pipelines ever built, an intrastate line in Ohio, was closed down after railroads responded by organizing highly efficient unit trains. That's the system working the way it's supposed to, pushing costs down and productivity up.

The question of water has been raised repeatedly in the long debate over this legislation. To move coal through a pipeline, it has to be crushed and mixed with water to make a slurry that can be pumped. That raises concern among farmers in the western states, who see the pipelines as a different kind of competitor -- a competitor for water. But Congress' Office of Technology Assessment looked into it and concluded that adequate water is available. Legally, the legislation would leave the allocation of supplies with the states.

Since this bill would encourage and broaden competition, you might think that the Reagan administration vigorously supported it. In fact, the administration has resorted to a craven cop-out. It piously explains that, in deference to the sacred principle of states' rights, it prefers to leave the whole matter of eminent domain to the state legislatures. That amounts to opposing the bill, since any long line must cross three or four states, of which any one could block it.

The pipeline bill has been reported in both the House and Senate, but it's moving in slow motion, and the end of the session is getting very close. If Congress acts, or fails to act, the difference will be read in people's electric bills for years to come.