Castle Cary railway station is on the Reading to Taunton line 115.25 miles (185.48 km) south west of London Paddington and the Bristol to Weymouth line 47.75 miles (77 km) south of Bristol Temple Meads. The two routes share tracks between Westbury and Castle Cary stations and are both operated by Great Western Railway, which also manages the station. The station is 1 mile (1.6 km) north of the market town of Castle Cary and 5 miles (8 km) south of Shepton Mallet in a largely rural area of Somerset, England.

The Reading–Taunton line is a major branch of the Great Western Main Line from which it diverges at Reading railway station. It runs to Cogload Junction where it joins the Bristol to Exeter and Penzance line.