Participating countries: the ITTL earlier version of https://en.wikipedia.org/wiki/League_of_the_Three_Emperors - i.e. German Confederation + Austrian Empire + minor interested German states. The yet not conquered countries and territories in Central and East Asia would be made by positive and negative stimulae to take part and this will literally pave the way for RuE faster conquest.
Such TEAR would result in concentration of RuE colonization and settlement hundreds of miles south then IOTL leaving Siberia more or less empty...
RuE might be interested from military ( defensive ) considerations to be happy to keep the line off Moscow and St.Petersburg. ( The major political reasons why RuE delayed the introduction of RW mode of transport IOTL ).
Lets make this TEAR 2-lined and really gargantuan , similar to OTL 20th century Hitler's RW dreams : http://en.wikipedia.org/wiki/Breitspurbahn - the Hamburg - Rostov section is geographically logical...