My little red caboose or cabin car in Pennsylvania Railroad terminology. Bobbers were simple wooden frame cabooses built in the 19th century. PRR made theirs with steel frames. That permitted their bobbers to be used in pushed service without being crushed. Their choice to use steel allowed their boobers to last well into the 20th century.
PRR Bobber Caboose by dr_spock_888, on Flickr
They were called bobbers because 2 axles gave a bobbing ride. I guess the modern ride equivalent would be of the U.K.'s Pacer railcars.
Bobbers have a child's toyish feel to them. Maybe because many little children toy caboose designs are based off them.