Shuttle with realistic slowing and acceleration

[@heathcote-electronics]

D1, D2, D3 and D5 refer to terminals on the SA1 board. We make other shuttles, the SA1-S which has control of a starter signal at each end of the line. The SA1.1 and SA1.1S which are like the SA1 and SA1-S but have the additional capability for one or more intermediate stops. The SA9 and SA9-S which have a siding at one end of the line so that 2 trains shuttle alternatively. The SA2 and SA2-S which have a siding at both ends of the line. Our website is at www.heathcote-electronics.co.uk