In the simple scenario the reed contact will close only momentarily while the magnet under the train passes over the contact. It will open as soon as the magnet has passed the contact. This will result in a short pulse only, the duration of which will depend on the speed of the train (slow train, long duration and vice versa). If that is enough for the gate to open you're o.k.
If not, you may need a timer module (
example) to generate a pulse long enough to open the gate, The timer would be triggered by the short pulse from the reed contact.