Jackboots were Snowpiercers Security Forces, acting as SWAT units. They were under the command of Commander Grey, former SAS member. They were seen not only guarding the tail, but also escorting prisoners and maintaining order and security for VIP such as Melanie Cavill when she visits certain parts of the train.
History
Formed from Security Forces that guarded the Wilford Industries' Chicago Compound and Snowpiercer itself, they were instrumental in stopping more stoaways, later known as "Tailies", getting aboard. However, they were ultimately unsuccessful in pushing them off the train, managing to only lock them in the last 3 cars[1].
All but few Jacboots were killed during the Civil War. Most notably over 130 members were killed when Melanie Cavill and Andre Leyton disconnected 7 cars from the Snowpiercer and left them to freeze to death on a siding[2].