Let the current speed be Xkm/h and the current running time be 312/x.

The design speed is 2.5 X, and the running time of Hening Line is154/2.5x.

Then: 312/x-154/2.5x = 3.13.

To solve this equation: x = 80.

Then the design speed is 2.5x = 2.5 * 80 = 200.

In other words, the design speed of Hening Railway is 200 kilometers per hour.