Hi Mick
From your description I assume that you are using the standard Auran couple command which requires you to couple to a named vehicle?
I use the two commands "SetDCCThrottle" kuid2:131986:1018:1 followed by "CoupleAhead" kuid2:131986:1015:2, both by atilabarut. You set the throttle speed with the first command (0.18 is about 3mph) and the loco/train will approach the consist and couple to it, stopping when it makes contact. If the train is a long distance from the consist that it is to couple to then you may need to Drive or Navigate via a trackmark nearer to the consist followed by the two above commands as, once the throttle is set the train will drive at that speed until it couples. These commands will cause the train to couple to the first vehicle that it comes to so there is no need to use the vehicle name in the command.
There are some other commands such as Couple at Trackmark which work in a similar way but I haven't used them as the two that I have mentioned do what I want.
Regards
Brian