Sum

MSAYED26 OK, try this one. Updated PBIX attached.

Travel Time Measure = 
    VAR __Departure = MAX([Departure station])
    VAR __Arrival = MAX([Arrival station])
    VAR __Line = MAX([Line])
    VAR __DepartIndex = MAXX( FILTER( 'Table', [Line] = __Line && [Departure station] = __Departure ), [Index] )
    VAR __ArrivalIndex = MAXX( FILTER( 'Table', [Line] = __Line && [Arrival station] = __Arrival ), [Index] )
    VAR __ArrivalIndexReverse = MAXX( FILTER( 'Table', [Line] = __Line && [Departure station] = __Arrival ), [Index] )
    VAR __Result = 
        SWITCH( __Line,
            "Blue", SUMX( FILTER( 'Table', [Index] >= __DepartIndex && [Index] <= __ArrivalIndex ), [Travel Time (min)] ),
            SUMX( FILTER( 'Table', [Index] < __DepartIndex && [Index] >= __ArrivalIndexReverse ), [Travel Time (min)] )
        )
RETURN
    __Result

Also, I don't think that your data posted is correct or something is weird. Red S1 goes to S2 to A4, to A3, A2, A1, A10 so I don't see how S1 to A10 is 12 minutes.