“Bogie” in British terminology, specifically refers to a two-axle or three-axle truck (no British loco types ever used 3-axle trucks, but some carriage stock did).
A leading, single-axle truck is a “pony truck” while the one under the firebox is the “trailing truck” - unless it is a radial axle, or Cartazzi type axle, in which case it would be so described. Fixed leading or trailing axles, not common but not unknown either, were simply known as “carrying wheels”