16603/04 are being short terminated at Kochuveli for a month due to apron works at Thiruvananthapuram Central. KCVL is a station 7 or 8 km short of Trivandrum Central and you can travel with same ticket if booked to TVC.
You can take any other train from KCVL to TVC(but availability is very less as only passengers halt there & the platforms where trains terminate and passengers leave are pretty far away though connected by FOB). Else you can try buses from outside station which usually ply coinciding with arrival of long distance trains).