There have been condenser locomotives, and they indeed do have double the thermal efficiency than the ordinary double acting steam locomotives. They have one major drawback: the condenser required is HUGE. The condenser really doesn't matter on ships or stationary power plants, but trains have these things called "payload" and "load gauge."
Hauling the condenser is about the same as hauling one extra fully laden freight wagon, and the condenser needs to be designed to fit within the load gauge. Moreover, on ordinary steam engines, the spent steam is fed into the exhaust to improve firebox draught, but condenser locomotives need an extra turbine to do so. Condensers could be heavy as well, which reduced the payload able to be hauled.
Condensing the steam meant the added complexity, and maintenance, of the fan needed to feed the blast pipe that pulled air through the firebox. Unless there was a compelling reason, it was easier to take on water at a station stop rather than add the complexity of condensers. Diesels replaced steam primarily because they required far less maintenance and they offered the flexibility of allowing multiple locomotives to be controlled by the one driving crew. The diesel electric locomotive had about 10 times the efficiency of the regular steam engine, and the condensing system couldn't compare even if it doubled the steam engine's efficiency.
Nevertheless, condenser locomotives have been used, and they have proven worth the extra investment in regions where distances are long, where water is sparse, and where fuel is expensive. Another extra benefit is that they do not leave the telltale vapour clouds behind them. The South Africans built 90 of these condensing locomotives from 1953 to work arid regions like the Great Karoo. Steam would be recycled up to eight times, giving the locos a 500 mile range. They had effectively sevenfold the range of ordinary steam locomotives, with no need to stop to replenish fuel and water. Deutsche Reichsbahn also used condenser tenders in Russia during the WWII, where distances are long.