I've never experienced this runaway situation with any of my Android devices and I don't have any Apple device to test this on. I've had runaways & horn lockup before but they were a caused by degraded DCS signal and before I was using the app.
Does the speedometer on the app screen increase as the train increases in speed?
Can the DCS remote override and regain control of the runaway train?
Can another device running the app override and regain control of the runaway train?