I think I read something like only 25-30 per cent of container traffic from Felixstowe and Southampton go by rail, plus there's virtually no/little container traffic from the likes of Liverpool, Bristol and northern east coast ports. I know Felixstowe has capacity constraints on the line into the port but its shows there's still a very big market there for rail to go for.
In theory yes. However the break-even distance for rail to be competitive on a box move is quite long. If a truck has to be sent to a container terminal it doesn't cost much more to send it all the way to the port, at least if that can be done and back within the driver's working shift. The problems for rail are further compounded by the need to run to a timetable that doesn't match the ships, so that even if the train journey itself is faster a truck will probably be quicker overall. Coastal shipping is also an option for less time-critical consignments. So for containers rail is competing on price rather than speed (this probably explains in part the 75% real terms fall in rates since the 60s mentioned in Modern Railways).
As a first approximation the origin/destination pattern of containers is likely to match the distribution of population. For Felixstowe and Southampton the densely populated parts of the South-East mostly lie within the out-and-back limit. The rest of this region, along with the south-west, doesn't have enough people to generate enough boxes to fill many regular trains.
Thus the main market for rail haulage of marine containers is the populated areas of the Midlands, northern England and Scotland, and it is no surprise that nearly all the trains take this route. Taking out time-critical consignments and the slice of the remainder going to areas where rail can't compete, rail is probably close to its ceiling at Felixstowe and Southampton.
For ports further north the situation is reversed, with Northern England and much of the Midlands too close for rail to be competitive but some opportunities to the South East and Scotland. However these are the same routes already used by Felixstowe and Southampton traffic. In the case of the South East there is also the shortage of terminals, the many problems of creating new ones, and the fact that a terminal in a suitably "central" location will be difficult for road vehicles to access due to traffic congestion. Bristol probably has the least potential for rail of all the major ports, with a large slice of the population accessible out and back within a truck driver's shift.
The overall market for containers has been growing strongly in recent years but one has to question how long this will continue, given that the capacity of the UK to absorb Chinese goods must find a limit sometime. Then the rate of increase in container numbers may be little more than that of the population. Rail may gain a little more as congestion and driver hours restrictions make road slightly less competitive, infrastructure improvements feed through, and individual flows increase enough to make economies of scale more significant. But I personally think talk of marine container numbers on rail doubling is illusory.
The opportunity may be more in domestic freight, but that's for another post.