A recess in the side of a partition of a passenger car to receive the faucet of a water-cooler or water-pipe and a drinking-cup. – The Car-Builder’s Dictionary, 1888.
She turned the handle just a little, and water came out of the spout. She turned the handle back, and the water stopped. Under the cup there was a little hole, put there to carry away any water that spilled. Laura had never seen anything so fascinating. It was all so neat, and so marvelous, that she wanted to fill the cup again and again. But that would waste the water. So after she drank, she only filled the cup part way, in order not to spill it, and she carried it very carefully to Ma. – By the Shores of Silver Lake, Chapter 3, “Riding in the Cars”
Where Laura got the tin cups full of water on the train was called the water alcove. It was a built-in recess in the side of a partition of a passenger car through which passed the faucet of a water cooler, a waste water pipe, and to which a drinking cup holder was attached directly beneath the faucet. The alcove was usually made of metal and had an ornamental guard across the bottom, which mainly served to help keep the cup from falling out of the cup holder when the train was moving. On some trains, the water tank was filled with ice water and insulated with wood. The waste pipe typically carried water onto the tracks below, through a hole in the floor.
water alcove (SSL 3)