MilepointRoute2015 shapefiles have I-781 ending at US11, with the road east of there, to the east end of the RR overpass, labeled as SR971Q.
TDV and
2014 TDR ("EAST END OF BRIDGE OVER RR END RT 971Q") concur; the TDR also lists I-781 up to "US11 & 971Q". Signage at US11 also shows I-781 only to the west. Notwithstanding the END sign being
after the exit ramp, it looks pretty cut & dry what to do here.
And why encourage civilians to go questionably close to military bases where their access is restricted, just to get a clinch?
I'll put in a pull request to truncate.