It sounds like your Hymek is an old Hornby Model where the motor is directly connected to the chassis and hence requires only one wire to complete the circuit.
These motors can be converted to DCC but are trickier to convert because the motor has to be isolated from the chassis which will involve replacing the metallic screw(s) holding the motor to the chassis with non- metallic screw(s).
I have had a quick search of the internet to see if I could find an illustration for decoder installation for this situation but nothing came readily to hand - but I know there will be something out there somewhere as I have read and seen posts on this subject before
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