Chippenham to Trowbridge - comparison of timings by public transport (with a private car thrown in).   Slowest journeys first, looking at the next departure by each mode from now (as I compile this):

16:34 direct bus from Chippenham, arrive Trowbridge 17:24 - 50 minute journey
16:45 train from Chippenham, change at Bath, arrive Trowbridge 17:28 - 43 minute journey
16:40 by car from Chippenham, arrive Trowbridge at 17:20 - 40 minute journey
19:01 train, direct to Trowbridge arriving there at 19:20 - 19 minute journey