The 5 bar mark is quite rare, I think. According to SG, it was used for railway mail during 1945/55 and also as a paquebot at Jesselton around 1950. Others has suggested that this was a cancellation used on the stretch of railway between Beaufort and Jesselton in 1948. Proud also mentioned that it was used for "loose mail", late letters etc.
Update I have recently seen a clear strike of this 5 bar within an oval cancellation on a 5c 1964 NB overprinted Sabah stamp which throws the above dates into dispute.
The "TRAIN MAIL" postmarks are relatively common and were in use from 1932 until 1942 when the railway was extensively damaged in the war.
The "TRAIN P O" postmark on the 1c tapir was in use between 1916 and 1923 is more difficult to find. It comes in 2 sizes, the earlier version being slightly smaller.
There are 9 or more different railway date stamps known for North Borneo. They were variously in use between 1916 and late 1970s.
Update Added stamp with a nice clean strike of "TRAIN MAIL" on 2c Travellers' Palm.
Update Added the 3c railway green with a "MAIL TRAIN" single ring postmark. This was used in 1920s to early 1930s. They are not common.
Update Added the 3c Murut with the larger version of "MAIL TRAIN" postmark. They must be less uncommon than I thought.
"MAIL TRAIN" cancel, this time in blue.
"TRAIN P O" cancel again in blue.
"TRAIN P O" in black. I thought it was a GPO at first glance.
Another "MAIL TRAIN"