Replace the trackpad cable. Commonly fails in 13" 2015 models with no explanation and costs £10, very easy to replace too. When this part fails, the keyboard also stops working, and it usually acts intermittently (both keyboard and trackpad sometimes working, sometimes not).
I guarantee this will fix your problem, it is not a logic board issue. I have answered this question a lot on iFixit (a lot of which have confirmed it has fixed the issue, if they did try out the fix), and receive a lot for repair with this same issue, which is always caused by the cable.