Newton's Law Problem, involves n-t frame or r-t frame

[jlopez]

After picking up passengers, a train accelerates uniformly until its speed is 100 km/hr at t= 300 s. If during this time it is traveling on a planar path that approximates the circle and line shown in the figure, find the magnitude of the train's acceleration when (a) t = 1 min and (b) t = 2 min.

link to figure:
http://i22.photobucket.com/albums/b346/lopezjose/IMG00264-20100502-0017.jpg

I'm not sure what frame of reference to use and how to start this problem

normal acceleration is v^2/r
tangental acceleration is ds/dt

 

[willem2]

Find out how far the train traveled along the tracks after 1 and 2 minutes.
If the train is still on the circular part of the track, then you have to combine the tangential and the normal acceleration. If it's on the straight track, there's only the accelaration along the track.

 

[jlopez]

do i use the 60 degrees to find the arc length of the circle?