This group of vehicles were variously lettered 'W TANK' or 'WATER TANK'. There is some confusion because the wagons are placed in oil tank number group.
Steam engine tenders were placed into oil tank service for some years. The tenders were generally coupled in pairs. Later, these units were placed into water tank service. They were used as standby water for fighting trackside fires. Most main country loco depots had one or two pair.
The only other type of 'W TANK' in use was a four wheel wagon with cylindrical tank. These were easily identified with numbers higher than 236. The number group was scattered as they were introduced simultaneously with fuel tank wagons.
Some numbers recorded by the author were: 460, 462, 463, 466, 467, 474 - 482, 497 - 501, 531, 532, 597 - 601, 606 - 615, 617 - 624.
The other group of _WT_ are for 'Water Trucks' numbered 1 - 236.