You will get this on the first task after the tutorials finish. London to Reading is 1 stop - there are no stops on this journey to worry about.
IMPORTANT: if you exceed the speed limit by too much or for too long you will fail.
Get on the train at Ealing Broadway into London and walk / run round to the HST. Enter the drivers door and follow the steps to get the doors open for passengers. You should have about 10 to 15 minutes to wait.
One the departure time is reached - accelerate the train. As you leave the station you will see green and red symbols. Green symbols indicate signals - its all green on this journey till you get to breaking into Reading. The Red symbols are speed limits for that section of the track - do not exceed these. The first speed limit is 50. What I did was accelerate the train to 50 then when I got to 50 shift the motor to Idle - and let it coast. When you drop a bit below 50, give it a little throttle Once you pass the red 50 symbol another one will appear for 100 - Accelerate to 100 and repeat the Idle - Throttle method. Then pass 100 and you get the 125 symbol. Accelerate to 125 and continue to use the throttle and idle to keep the speed around 125 until the breaking for Reading about 20 minutes later!