As the height of the sailtrain ramp is more important as the length, height/speed ratio could be much like a ski-ramp. What I know for sure is that a straight 90° drop would give an acceleration of 9.8m/s/s  

We assume our desired speed is 100 km/hr OR 27.7777778 m / s

the drop would take 2.83446712 seconds.

The average speed would be 50 km/hr OR 13.8888889 m/s 

13.8888889 m / s * 2.83446712 s = 39.3675989 m 

To reach our desired speed we need about 40 meters height (or depth).

Although there is no official definition, a minimum height of approximately 150 metres or 500 feet is often used as a criterion for a building to qualify as a skyscraper. The Taipei financial centre in Taiwan is 508 m.  Current ideas for roller coasters are stuck around 400-500 meters.