When I press the left stick (L), both the siren and the train bell are activated together. Xbox one s version.
It's like that on all US trains on all console (at least) versions, when you press the horn it activates the bell as well. Same on Sand Patch Grade or on Peninsula Corridor. Only LIRR doesn't have it but that's because there's no bell there. So I think it's not a bug but a feature.
To disable the bell press the bell button in the cab or either the up or down arrow on the D-Pad (one is for the bell, one is for the emergency brake)