Analysis of railway data - Detecting outliers
There are a number of directions you can take, and there are a number of questions related to this topic (anamoly-detection-for-transaction-data).  One question to answer first is whether the data is seasonal or not.

You already suggested a valid approach.  Determine which areas are correlated to low and high speeds and than determine when a speed differs.  I think it's best to start with a heuristic approach and than try more complicated methods to try to improve.  If you want to use an SVM, a one-class SVM may work well (one-class SVM)