ETCS Balise groups

[Rob]

When can balises be placed by themselves (i.e. in a balise group containing only one balise)? 

https://en.wikipedia.org/w/index.php?tit...=894294214 states that "Eurobalises are typically placed in pairs" and that "Singular balises exist only when linked to a previous balise group or when their function is reduced to provide only the exact position."

A single balise is still referred to as a balise group. 

Reasons to have more than one balise in a balise group include:

• Directionality - when a train's onboard unit (OBU) passes over a balise group containing more than one balise it can immediately infer if it is travelling in the normal or reverse direction relative to the balise group and can therefore decide whether the information in the telegram's packets (its data) applies or not.
  
• Redundancy - telegrams can be duplicated in separate balises reducing the chance a failure results in the train not receiving the intended information.
  
• Higher data capacity - a single balise can only contain a certain number of bits of data, more balises (up to 8) may be used to transfer all the required data to the train.
  

However, as far as I can tell, in many situations in a level 2 system a single balise would suffice. Balise linking allows  the OBU to be told of balises it can expect to be read beyond its current position. This linking information allows the train to deduce its direction in relation to a single balise based on a history of at least one other balise group (that may also only contain a single balise). So as long as the OBU has passed over two linked singular balises it knows its direction. In level 2 balises normally contain little information - mainly allowing the train to understand and report its position in relation to a fixed location on the track (the balise).

Cases where more than one balise may still be required (ignoring redundancy and data capacity):

• Where trains enter the ETCS area (hitting the first balise and needing to know their direction) or wake up
  Including exits from sidings, depots and maybe terminus platforms etc
  
• Maybe balises placed to trip trains in 'staff responsible' mode e.g. at the end of authority at a junction. (presumably this function would still work on a singular balise - but only if the train has the linking information).
  

So in a level 2 system the majority of balise groups could have only a single balise? What have I missed; why are balises 'typically placed in pairs'?

(Asking for general knowledge rather than directly for the exam - not sure if this is the best place to post this question)