Re. Gronigen Expressflight,
You quote an AMS/GRQ/AMS round trip rail journey time of 4.67 hours and you cite that this makes life, so to speak, difficult for day returns.
LON/GRQ/LON. Just how many punters actually want, will utilise, round trip day returns LON/GRQ/LON? ... I would suggest very few!
But serve AMS then there will be all the punters connecting to KLM, and others, internationally whilst connections may also be offered to/from GRQ.
Is a night in a GRQ hotel such a hardship for a corporate businessman ... your reasoning of a SEN/GRQ/SEN route is a very weak reasoning to say the least!