Tom and Andrew,
The 400 series numbers for the SP narrow gauge cabooses were assigned post WWII. I believe that this goes along with the renumbering of the freight cars that also occurred post WWII in the late 1940s.
And Andrew is correct the 400 disappeared -- I believe in the early 1950s. When the SPCRR group from Ardenwood acquired the body setting in Keeler they thought they had obtained the 400. But in reality it was the former SP 455 which had been retired in 1915!
The South Pacific Coast had four of these Carter built combines which they used as cabooses -- 45-47
45 Caboose -- unknown
46 Caboose -- vacated prior to 6-30-06
47 Caboose -- sold 10-27-07 to N&C; to N&C caboose #455 10-23-07; worn out, 11-28-15; body to Keeler, Calif.; body moved to Newark, Calif.
48 Caboose -- sold 10-27-07 to N&C; to N&C caboose #456 10-23-07; changed to boarding car MW 456, 1-9-10; changed to caboose #1, 6-1-18; later #400; retired
I hope that helps.
Brian Norden