Multi-Terminal Binary Decision Diagrams (MTBDDs) have been successfully applied in symbolic model checking of probabilistic systems. In this paper we propose an encoding method for Probabilistic Timed Automata (PTA) based on MTBDDs. The timing information is encoded via placeholders stored in the MTBDDs that are independent of how the timing information is represented. Using the Colorado University Decision Diagrams (CUDD) package, an experimental model checker is implemented, which supports probabilistic reachability model checking via the forward algorithm. We use Difference Bound Matrices (DBMs) and Difference Decision Diagrams (DDDs) for representing timing information and present experimental results on three case studies. Our key contribution is a general placeholder encoding method for Probabilistic Timed Automata and an experimental MTBDD-based model checker which has been partly integrated with PRISM. This is the first symbolic implementation of the forward probabilistic reachability algorithm.