As you know, the first (?) railway terminus in this state is Naharlagun which serves the capital Itanagar. There is also an intermediate station Gumto between Harmuti Jn (in Assam) and Naharlagun.
For a long time it used to be said that Bhalukpong was the first station in the state. While Bhalukpong town is in Arunachal Pradesh and spills in to Assam, the station is in Assam and just short of the border. That is what Google Maps shows.
However, there is no Assamese inscription on the sign, which would be there if it was in Assam. Also note that the sign says Bhaluk Pong (2 words) in English and Hindi.
It may be more correct to say that Naharlagun is the first important station in the state, as traffic to Bhalukpong was generally low and was suspended for long periods.
However, the first railway line to be laid in Arunachal Pradesh is a stretch of about 500 M between Dimow and Dipa on the Dhemaji-Murkong Selek section. It is not known whether the state government keeps an eye on infiltration on this route.
The first major station:
And the pretender: