I try to get some of the systems here, but I'm afraid I don't remember every of them.
Most of basic MUs had two letters and a number as class: the first letter was D or E, depending on being diesel- or electric-powered, and the second one meant the place of manufacturing (R = Riga, D = Dyemidovo, etc.). So e.g. ER2 and ER9 classes mean EMU classes made in Riga, DT1 is a DMU from Torzhok, etc.
Electric locomotives built in Novocherkassk had classes starting with VL (after Vladimir Lenin): VL8, VL10, VL80, etc. - locos with small numbers were DC, and with high numbers were AC. The continuation of the VL65 series became EP1.
The common diesel locomotive built in Lugansk (Voroshilovgrad) for almost all countries in the Eastern bloc had the M62 class after the Hungarian system (because they bought it first). The Soviet ones started uniquely from M62-1001 (all other Soviet locomotives started from 001/0001...), and the twin and the triplet versions got 2M62 and 3M62 class.
The Hungarian shunting locomotives had a V (after "vengersky"/Hungarian) as first letter, the second was M ("manyerovy"/shunter).
The Czechoslovakian-built locomotives have a Ch (after "chekhoslovatsky"/Czechoslovakian) as first letter, independent on being electric or diesel-powered.
Almost all Soviet-built diesel locomotives had TE (after "tyeplovoz"/diesel locomotive) as first characters. After that, another letter meant the field (P for "passazhirsky"/passenger, M for shunter, etc.). The earlier mentioned EP1 follows this structure (E for "elektrovoz"/electric locomotive, P for passenger).
And there are lots of extra secondary letters with the different modifications which can mean anything you can imagine (e.g. m and k in ED4mkm, etc.). They all follow the original class, starting after the number part.
