Compared to the weight of a rider, I'd expect the bagman to exert negligible forces on your saddle rails.
@Bearlegged started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Compared to the weight of a rider, I'd expect the bagman to exert negligible forces on your saddle rails.