As shown in the WAGR outline diagram above, the underframe used in these wagons was the 30'-0" underframe also used under the Q, R, S, T, V and Y class wagons although there were some variations with the bogies and braking arrangements. As delivered the U class wagons had no sides but had two-plank ends and pockets along the side for six large stakes. These side stakes seem to have disappeared very quickly and special bolsters were fitted to many U class wagons to allow them to be used to transport pipes for the Goldfields water supply scheme. In their early days 690, 694 and 2978 were used to transport boilers. In 1898 the shortage of water in the Goldfields resulted in wagons 2981, 2987, 2988, 2990, 2993, 2998 being fitted with large square iron water tanks and being reclassified FB. In 1900 2981, 2988, 2993, 2998 reverted to class U and the class was joined by 3521, 3521-3530 converted from class FB which had originally been bogie opens (IB later R). There were further conversions over the next few years:

1904/5	689, 692, 693, 2977, 2978, 2979, 2981, 2984, 2988, 2994, 3201, 3204, 3205, 3209, 3210, 3212, 3215, 3218, 3223, 3224, 3228, 3242, 3243, 3248, 3523, 3526, 3528, 3529 converted to class Q
1906	690, 691, 2976, 2983, 2985, 2986, 2991, 2992, 2993, 2996, 2997, 2999, 3000, 3202, 3207, 3208, 3211, 3216, 3217, 3219, 3221, 3222, 3226, 3227, 3230-6, 3238, 3239, 3240, 3244-7, 3521, 3524, 3525, 3530 converted to class Q 3203 written off (presumably in an accident but see below)
1908/9	2982, 2998, 3200, 3206, 3213, 3214, 3249, 3527 converted to class JA

A little mystery surrounds wagon 4897. In 1900 this wagon was listed as a U class along with a few other odd wagons in a series which appear to be ex contractor wagons and so could have originally been 3203. By 1909 it had been converted to class Q. Presumably the wagons were felt to be more generally useful as Q class bogie bolsters than as U class platform wagons once the Goldfields water supply scheme was completed. The conversions to class JA in 1908/9 were a repeat of the conversions to class FB ten years before. A further wagon (2989) was written off in 1920 but then the ranks of the U class wagons started to recover with wagons converted back from class JA as follows:

1923	2998
1924	3206, 3213
1925	2982, 3214, 3249

In 1925 a further three wagons (705, 710 and 744) were obtained from the Water supply department. The origin of these three wagons is not clear to me at present. In 1933 wagons 2980 and 3241 were extensively converted to carry equipment for spraying weed-killer. Number 3241 lasted until 1968 and was then written off but in 1966 number 2980 along with 710 and 3229 were converted to carry apple containers. WAGR official photo of Weedex wagon U3241 in action. (RHWA collection) Below: WAGR outline diagram.

In 1939 number 705 was converted to an R class open wagon whilst the following year numbers 693 and 2981 were reconverted from class Q to class U and then sold to the Commonwealth Railways where they became class NRB. One of the last U's in traffic was 1730 which was used as a runner for a crane. The last wagon converted to class U was 1730 (formerly of class R) which was used as a runner for breakdown crane 23. It replaced U694 on that job. The remaining wagons were written off as follows:

1947	3214 to JETTY 487
1950	3220 written off
1964	694 written off
1970	744 to JETTY 13
1971	710, 3206, 3213 written off
1972	2980 written off
1973	3249 written off
1975	2998 written off
1979	1730, 2982, 3229 written off

In practice number 1730, although written off, was still in use with its breakdown crane when the crane was preserved in 1990 after being officially written back in 1981.

One other wagon carried the U classification. This vehicle was a one-off conversion on the isolated Port Hedland Railway of SX class sheep wagon 10235 in 1933. The conversion may have actually happened before this as the photo below is dated to 1928 at Port Hedland but dates on photos can be unreliable. It was 33'-0" long and was never used on the main system. It lasted until the Port Hedland line closed in 1951 and was then transferred to the Public works department.