Bus & train transport.

Are dogs allowed on either of these public transports.

On trains: yes bdz.bg/p.php?id=pets (I found it only in Bulgarian but Google translates it very well)
On buses it's rather a matter of negotiation with the driver. On village to village buses probably there won't be a prolem, but if it's a Sofia - Varna/ Plovdiv/ Tarnovo/ Burgas/ Ruse bus, I don't think they'll like the idea.

Thank You Alheratz, It is slightly better than Portugal where dogs are not allowed on buses and trains, you have to show there passport.

peterb25