I was thinking it was far more likely to be a train service, but there is no real way of knowing. Those prices are far too high for a regular train ticket bought sufficiently in advance. The frequency of the service - almost 1 per hour it seems on most days - may mean it could be a train service - but there is no direct service from AMS/Schiphol to DUS so I am not at all sure what this relates to.

It doesn't appear in any way a KLM initiative, so I don't think that this is related to KLM moving shorthaul to trains.

(A full-flex economy train ticket on the same dates as I showed above for these "flights" can be had for less than half the price - but take more than an hour extra)