This work develops a real-time algorithm to localize a vehicle in the direction of travel without the use of GPS. The inputs to the algorithm include a terrain map of road grade and pitch measurements from an in-vehicle pitch sensor. Localization is achieved in real-time using a particle filter described in detail in this work. Simulations and experiments at The Pennsylvania Transportation Institute test track are used to demonstrate the algorithm, observe the speed of convergence, and to determine key parameters for practical implementation. The results indicate that the method can quickly localize a vehicle with one-meter accuracy or better.