Even the original Half-Life monorail intro sequence had the player moving relative to a moving vehicle.
You weren't moving on a simulated rigid-body physics object, but in practice there's not much difference - other than handling the nasty cases, like what to do when the physics object flips upside down and crushes the player, or resolving collisions with static world objects that the player makes contact with but the 'vehicle' doesn't.
In HL1 you fly to the back of the tram when you jump because you only move with it while grounded. In HL:Source you move with the tram correctly and jump in place relative to the tram. Bafflingly in Black Mesa it's back to the incorrect HL1 behavior. Always bothered me.
You weren't moving on a simulated rigid-body physics object, but in practice there's not much difference - other than handling the nasty cases, like what to do when the physics object flips upside down and crushes the player, or resolving collisions with static world objects that the player makes contact with but the 'vehicle' doesn't.