Many upgradeable engines with Signalsounds of more recent vintage are fairly easy to update to RailSounds by opening up the unit and swapping circuit boards.
If it's the one I think it is your PA-1 also had another option as well. Lionel sold a PB unit (6-18965) that was fitted with RailSounds from the factory. The idea was to switch off the SignalSounds on the PA and let the RailSounds in the PB do the sound honors.
I have the NYC versions of both. They work well together, especially if you program both with the same ID.
Mike
