Does anyone know how long a trademark / logo has to be abandoned before it can be used by someone else? I am thinking of using logos of defunct railroad companies - in one case the trademark is shown in TESS as "Dead" and has not been used since 1978 for any reason. In another case the company issued a final liquidating dividend and dissolved in 1943. Its logo has been out of use since 1940.

Trademark registration is only notice that a mark definitely is in use. It cannot tell you that a trademark is available for use. The "Dead" designation only goes to whether constructive notice exists as a matter of law. To learn whether you can use a "Dead" mark you will need to do the same level of due dilligence as if the mark had never been registered at all. That would mean searching the internet for uses of the mark, searching all US state offices that recognize registered marks. Searching for companies in the same line of products as you want to use the mark for competing uses.

Also just because a trademark is dead does not mean that the artwork that constitutes the logo is unprotected by copyright. In many cases the logo will be under copyright. There are many factors that will need to be investigated to determine whether copyright applies.

http://tutortanith.com/legal/content.htm#law

Trademark may not be the the only issue. If the logo is more than just simple plain text then someone owns the copyright to that design.