As remote signalling systems were invented, the workmen were replaced with signal towers at set locations along the track. This broke the track into a series of "blocks" between the towers. Trains were not allowed to enter a block until the signal said it was clear, thereby guaranteeing a minimum of one block's headway between the trains. This had the side-effect of limiting the maximum speed of the trains to the speed where they could stop in the distance of one block. This was an important consideration for the Advanced Passenger Train in the United Kingdom, where the block sizes limited speeds and demanded a new braking system be developed.