With the current Eurostar fleet, what's the highest intensity of service that could be run beyond Brussels?
Currently, the Paris route has a much more intensive service (up to 18 trains per day in each direction) than the Brussels route (up to 9 trains per day in each direction). Demand for travel to/from Paris is far higher than Brussels.
Once some of the Brussels services continue to Amsterdam, providing the service is a commercial success, this may start to even up the capacity required on each route. As I understand it, on Amsterdam/Rotterdam -> Brussels -> London journeys, half of the train will be allocated to Amsterdam/Rotterdam -> London passengers and the other half allocated to Amsterdam/Rotterdam -> Brussels and Brussels -> London passengers. This effectively halves the demand needed to make a viable service to Brussels/Amsterdam compared to operating to either destination alone. If the pricing is right, you'd think that Amsterdam could support at least 4 train in each direction per day from London.
If the Rotterdam/Amsterdam route is a success, Cologne/Frankfurt is the other route that's been mentioned previously as a potential expansion for Eurostar. Clearly needing to fulfil the UK government requirements re: security before boarding is a big barrier to getting this up and running, but would the Eurostar fleet availability allow for running a further 4 trains per day beyond Brussels to Cologne/Frankfurt? Alternatively, if the Rotterdam/Amsterdam route was more successful, could most/all of the current London -> Brussels terminating services run on to Amsterdam?
Essentially, how many of the current London - Brussels services could be run on to a destination a further 2 hours away (i.e. Amsterdam / Cologne) or 3 hours away (Frankfurt)? Could all 9 of the Brussels trains be extended to run beyond Brussels? I doubt there's this much slack in the current fleet availability, but if anyone knows (or has a good way of calculating this) it'd be interesting to hear!