In February or early March 1912 ancestors Gr Grandfather, wife, daughter, and son went to England and then caught ferry for a train to Germany. After visiting various family there returned to England to catch an ocean liner to the USA. However somewhere between the continental ferry and the train to the ocean liner they were delayed and fortunately missed the Titanic. Any idea what boat(s) they might have taken to / from England?
In 1912 they would certainly have taken the train from London to whichever port, then crossed by the ferry (as foot passengers) and then caught another train. There would have been several viable routes, really depending on the sea crossing taken.
The most likely route for Germany would have been from Liverpool Street to Harwich Parkeston Quay (built by the Great Eastern in the 1880s for this traffic), then the ferry to Hook of Holland (near Rotterdam), picking up a train again from there to Germany. Through cars would certainly have been run from Hook of Holland to Berlin and probably Frankfurt. This would have been the longest ferry crossing, but in a way that was an advantage, since the ferry was* scheduled overnight, making a cabin on the ship desirable for those who could afford it.
*and indeed still is - this remains a perfectly reasonable route for the traditionally-minded.
They might alternatively have crossed the channel at its narrowest point - via Dover or Folkestone to Boulogne or Calais in France, then on by train. These crossings were and are a lot shorter - about an hour and a half - but the train section would have been longer.
Since they missed the Titanic, my guess - but only a guess - is that they were on the Harwich route, on the basis that it might have been easier to recover from a problem if they were coming via France (there being more sailings on the shorter routes).
Both the Great Western (to/from Paddington to Plymouth and Falmouth) and the London and South Western (to/from Waterloo to Plymouth) ran Ocean Specials to connect with the transatlantic traffic, for passengers and - especially - the mail from New York. Getting the mail through in competition with the GWR may have contributed to the LSWR’s wreck of its Ocean Special at Salisbury in 1906. This happened on the very day that the GWR opened its cutoff from Castle Cary to Taunton, shortening its route by 20 miles.
Titanic sailed from Southampton. This was something of a novelty in 1912 - White Star had moved their main operations from Liverpool to Southampton only in 1907, and Cunard did not do so until 1919. There would certainly have been several special connecting trains from Waterloo.
If you add in the Irish Mail from Euston to Holyhead - for Dublin - boat trains of one description or another must have been quite busy in the early 20th century.