Hornby do not make a Next18 decoder so if you wanted to use a Hornby 8 pin R8249 basic decoder then you would have to carefully remove the Next18 circuit board from your loco (noting each of the 4 wires connected to Pick-ups and motor) and either solder these wires directly to the new decoder or, preferably, solder in an 8 pin socket. If your Bachmann loco has lighting then it gets more complicated.........HB