Down gradients will have a lower speed limit than up-gradients.
Usually, on a down gradient, the loco has to brake to make sure that the train does not over-speed. Now, on a down gradient, even if the LP does not accelerate, the train will accelerate automatically due to gravity, unless the driver is applying brakes. So, to ensure safety, trains are run at a slightly lesser speed than the MPS to give some room for error.
There are no such problems going uphill, so trains...
more... going uphill will have a higher MPS.