Trainspeed Indicators

The scale is set by how far apart you place the two optical sensors.  For the same scale speed, an HO engine travels roughly 1/2 the distance as a O engine.  So you'd place HO sensors half the distance apart as for the O sensors.  The manual provides a table for different scales.

The sensors detect changes in ambient light (as the train passes over them).  So it doesn't work in the dark.  This device only works at one location.  They offer a 4 input version that measures speed from 4 different pairs of sensors.

To each his own, but for the money I'd want one that measures speed anywhere on the layout which I suppose means it would be mounted on a piece of rolling stock sort of like the voltmeter-car.  There are DIY articles about mounting low-cost bicycle digital speedometers to a flatcar or whatever. You fabricate a mechanical sensor that generates some number of pulses (e.g., reed-switch closures) for each axle rotation.  The trick is to do the math on bicycle tire-size and the train scale to choose build the right sensor so the numbers come out in scale MPH.