ALOT CAN BE SAID foe passenger trains - and the letters printed today sum up what is being said in opposition to the Amtrak cutbacks that the Department of Transportation has proposed. Those readers are right in maintaining that energy problems strengthen the case for public investment in heavily used trains. But the current controversy is not about the Metroliners or the Empire Builder or other well-traveled routes. The issue is whether to continue the long-distance trains that consume large subsidies without drawing many passengers. Those losers, and those losses, are what DOT wants to stop.

Consider one train on the endangered list, the Floridian, which runs between Chicago and Miami or St. Petersburg. That might sound like a popular route. But the Floridian is very, very slow; its 1,597-mile "run" takes over 37 hours, including two nights aboard. (For comparison, the fastest train between New York and Miami, a 1,368-mile trip, takes one night less.) That at least partly explains why last year the Floridian ranked next to last in ridership and had the largest losses-per-passenger-mile of all long-distance trains. According to DOT, its costs in 1978 were $25.2 million. Fares covered only $4.7 million, about 19 percent. The federal subsidy amounted to $163.41 per passenger, more than the cost of a first-class plane ticket from Chicago to Miami.

Of course, almost all trains lose money. On the New York-Florida route, for instance, fares covered about 32 percent of the costs last year, leaving a federal subsidy of $98.55 per passenger. Those figures are not ideal. Yet when federal dollars are limited, it surely makes more sense to cut some losses and concentrate on improving the more efficient and popular routes.

The bill about to be considered by the House takes this approach. It would continue some of the trains that DOT wants to stop - if their ridership increases enough and their costs can be kept down. In an effort to keep the emphasis on performance, not politics, the House bill's sponsors have declined to speculate about which routes might make the grade. However, the latest figures suggest that even the surge in train travel will not help some routes enough. Instead of putting off the hard decisions, Congress ought to accept the fact that some trains have little future - and it's time to concentrate on those that do.