The Rio Grande Zephyr is the last old-time passenger train still running in America. It has waiters and white-linen-covered tables in the dining car, widely spaced seats, and vista dome cars where passengers can scan the spectacular mountain scenery along the route between Denver and Salt Lake City.
Despite its popularity with summer Dourists and railroad buffs, the Zephyr soon may join the rest of the nation's privately owned passenger trains in the history books. The three-day-a-week service has been a steady flow of red ink for its owner, the Rio Grande Railroad, which claims it is losing as much as $3 million a year. Because of the Zephyr's popular appeal, however, the Interstate Commerce Commission has refused to let the Rio Grande discontinue it. This month the company began negotiating with AMTRAK, the federal rail passenger line - and AMTRAK appears as eager to take over the route as Rio Grande is to discontinue it.
''Unfortunately, long-distance passenger rail service cannot compete economically with airlines and the heavily subsidized highway system,'' says Sam Freeman, Rio Grande vice-president and general counsel.
Should AMTRAK take over the route, it will use its newest and most luxurious equipment: double-decked passenger cars, a glass-topped lounge car, and a formal dining car. While comparable to the older train, which has been making the run since 1948, the new equipment lacks that Old West aura of the Rio Grande train.
AMTRAK believes it can profitably run the route. The take-over ''would let us duplicate the route of the old California Zephyr, the most popular passenger train in American history,'' says AMTRAK's Arthur Lloyd. It also would give AMTRAK access to Colorado ski resorts.