It's actually mostly flat, because it primarily follows the old canal route in Connecticut, and a former rail route in Massachusetts.
The trail's southern start in New Haven is just slightly above sea level. The highest point, between Southington and Plainville, is at an elevation of 310 feet.
In the chart below, you can see the two other high points, just south (left) of Avon and just south (left) of Southwick. These two points are also slightly over 300 feet elevation.