Visa requirements for Indian Nationals transiting through Schengen to the UK, by train/ferry?

I'm looking into the cost and complexity, for a colleague, for them to come over to the UK from India during August. Normally, I'd lean towards suggesting they get a UK visa and fly into Heathrow or 2nd choice Gatwick. However, because the Olympics are happening at the same time as the proposed trip, flying into London risks being a lot more expensive than usual.

Because of where they need to get to, flying in Amsterdam may well work out a lot cheaper, and then they could take the train + ferry + train to get themselves over. (They're heading for NE of London anyway, so arriving at Harwich is actually not really a lot out of the way). The cost of train + overnight ferry with cabin + train actually works out at a very reasonable price, especially compared with the costs of getting from Heathrow or Gatwick to the destination by train....

However, if they were to take this route, they'd need to leave the airport in Schiphol, in order to get the train to Rotterdam for the train to the ferry terminal. Since they're an Indian national, that could well prove a lot more tricky visa wise than for an EU passport holder.

Does anyone know what the visa situation would be for an Indian passport holder, in possession of a UK visa, and in possession of train+ferry tickets? Are there any waivers in place for transiting passengers who hold a UK visa? Would a normal Schengen transit visa work? Or would they need a full Schengen visa in order to be able to get to the ferry to come over to the UK? (Let's assume they'd basically go straight from the airport to the ferry, without stopping off, and the same on the return, so they'd be in the Netherlands for under 12 hours each way)