This paper explores the quantification and forecasting of reserve capacity from electric trains for participation in power system ancillary service markets. We first map train electricity consumption – traction and non-traction – to suitable reserve products, considering operational and regulatory constraints. Using historical data from the Danish railway operator DSB, we estimate the available flexibility for frequency containment reserves, focusing on controllable non-traction loads such as heating and air conditioning. To support market participation, we develop a low-resolution stochastic forecasting model based on conformal prediction, capable of estimating reserve availability for both day-ahead and hour-ahead horizons. Results show that a fleet of approximately 60 active trains can provide up to 10 MW of downward regulation and 1.5 MW of upward regulation from non-traction loads. Additionally, traction power from 25 trains can provide up to 5 MW of upward reserve in certain time periods. The findings demonstrate a viable pathway for integrating electric trains into flexibility markets, offering new revenue opportunities for operators and enhancing grid stability. • Demonstrates the feasibility of using electric trains to provide Frequency Containment Reserve. (FCR-D). • Introduces a forecasting framework using conformal prediction to meet regulatory P90 compliance. • Quantifies reserve capacity from real-world Danish railway data under operational constraints. (data specificity). • Shows minimal impact on schedules and comfort when participating in ancillary service markets. (market readiness). • Advances sector coupling by integrating railway operations into future energy flexibility systems.