The most significant difference between high speed passenger trains and freight services is – of course – speed. When passenger and freight services wish to share the same tracks, you have to consider both the technical aspects and the capacity issues.
When designing a new high-speed line, the minimum radius of the curves will be determined by the desired line-speed. In order to minimise rail wear, and provide a smooth and comfortable ride for the passengers, the outer rail of the pair of tracks is usually elevated above the level of the inside rail. This is known as “cant”, and in Europe it is usually measured in degrees, i.e. the angle of elevation of one rail above the other. In US terminology, “cant” is known as “cross level” and it is measured as a height difference, rather than an angle.
Put simply, the theoretical aim is to achieve a balance between the centripetal force of the train being pushed outwards around the curve, and the gravitational force pulling it inwards, at the speed at which the passenger train will normally transit that section of track. If the track designer achieves this, then the train ride will be smooth, and the wear on both the rails and the wheelsets of the train will be minimized.
It gets much more complicated if you add freight trains into the equation. Whereas the passenger train might be travelling at 300 kph or more, the freight train is likely to be travelling at 120 kph or even less. So when the freight train tries to go around this curve, the wheel flanges will be continually grinding at the lower rail head, as the gravitational force pulling the train downwards greatly exceeds the centripetal force, because of the lower speed. The result is significant rail wear, and the need to inspect and replace rails much more frequently.
Another technical issue is the design of the trackbed drainage. When building a new line, it is necessary to consider the consequences of any derailment which might release hazardous goods into the environment. So the drainage of the line has to include catch ponds to prevent any such spillage from entering the public drainage system. However if you design for passenger trains only, the issue of dangerous goods does not arise, so this element of the infrastructure design can removed from the plans.
When designing a line purely for high speed passenger trains, you can use steeper gradients because of the excellent power:weight ratio of the rolling stock. Gradients as steep as 3.5% can be used. This is too steep for heavy freight services, particularly if they are to run at sensible speeds (both for commercial and operational reasons). If the line is passenger only it can be designed to follow the contours of the countryside more closely, whereas a line designed for freight will require shallower gradients, and therefore higher bridges and deeper cuttings.
The big difference in speed between a high speed passenger train and a freight train means that trying to run both together on the same line at the same time consumes a huge amount of capacity. With a modern signalling system such as ETCS Level 2, you can run successive trains at intervals as low as 3 minutes, even at speeds in excess of 300 kph. Imagine you have a freight train in a siding, waiting to join the main line, with the next place it can be looped being some 50 km distant. A non-stop passenger train will cover that distance in just 10 minutes, whereas the freight train will take 45 minutes or more, maybe over an hour depending upon the weight, maximum speed, and type of locomotive. So to run a freight train you would need a gap of at least 40 minutes in the passenger service – maybe more. With the signalling enabling a path every 3 minutes, that means in this example 1 freight path equals 13 passenger paths! Would the freight operator pay for 13 passenger train paths in order to move his train? I don’t expect so!
You can overcome this to some extent by changing the use of the route according to time of day. For example, in Germany the Hannover to Würzburg line is used for passenger services between 05:30 and 23:00, and freight services overnight. Similarly in England the line from the Channel Tunnel to London is used by some freight services overnight, when no Eurostar international passenger trains or “Javelin” domestic express passenger trains are running.
One other option to consider is moving high value parcels traffic in specially-adapted high speed trains. In France, SNCF do this with their “TGV Postale” sets, which move letters and parcels traffic between Paris and Lyon at speeds which match the passenger services. Of course this requires investment in specialist equipment and rolling stock, but it does neatly overcome the issue of trying to obtain capacity for this specific type of freight during the hours of daytime passenger train operation.
So it is possible to run both freight and passenger on a high speed line? Yes, provided the line is specifically designed to accept them both from the outset, and provided the capacity can be split in such a way that the two different types of train have minimal interaction. However, running freight on a high speed line involves a significant number of compromises, some with big price tags attached, so it is not to be considered as a “free gift”.