According do the national Railway company’s website http://bdz.bg :

• in internal and international travels
  · Railcard ”?” – this pass is issued only against an identity card or an international passport and entitles the holder to buying tickets with a 50% reduction in price and passes with a 25% reduction in price while traveling within the country and to buying tickets with a 25% reduction in price in international travels. The price of the pass is 80 levs and has a term of validity one year after the date of issue.
• in international travels
  · Railcard “-26M” – this pass is issued to young people up to 26 years of age against an identity card or an international passport and entitles the holder to buying tickets with a 25% reduction in price in international travels. The price of the pass is 30 levs and has a term of validity one year after the date of issue or up to the date of the holder’s becoming 26 years of age.
  · Railcard “S” – this pass is issued to persons (women and men) over 60 years of age against an identity card or an international passport and entitles the holder to buying tickets with a 25% reduction in price in international travels. The price of the pass is 30 levs and has a term of validity one year after the date of issue.

Also:

• CityStar is an offer resulting from a two-sided agreement between the railway administrations of Bulgaria and several European countries, according to which travelers are entitled to significant discounts of the return ticket price.

• Inter Rail is a special rail pass for unlimited travel in 29 countries in Europe, divided into 8 zones. According to the conditions of Inter Rail there is no age limit, there are three price levels depending upon the traveler’s age: up to 26 years old – “Inter Rail –26”; over 26 tears old – “Inter Rail 26+”; up to 12 years old – “Inter Rail -12”.

• Euro Domino is a special discount freepass, which allows you to travel on the entire network of 30 European countries. Euro Domino pass offers you the opportunity of unlimited railway travel in the country of your choice during 3, 4, 5, 6, 7, 8 days as you choose, over a period of one month. You will have unlimited access to the entire railnet of any country participating in the offer, excluding your own country of residence. Euro Domino travelers are divided into three age groups: youths (up to 26 years old) with the right of second class travel, adults – 26 years old and above and senior – 60 years old and above with the right of first and second class travel. Euro Domino ticket holders are entitled to a discount of 50% on their way to the country chosen, where the ticket is valid.

Yep, you can use it – actually, Bg was the one place where my fake ISIC card (picked up in Turkey) was rejected by a ticket seller at a very small town station.