Here is the full list of all 25 Railbreak achievements.
Clear Act I
Clear Act II
Clear Act III
Clear Act IV
Clear Act V
Clear Act VI
Achieve Max Rank on Act I in Score Attack
Achieve Max Rank on Act II in Score Attack
Achieve Max Rank on Act III in Score Attack
Achieve Max Rank on Act IV in Score Attack
Achieve Max Rank on Act V in Score Attack
Achieve Max Rank on Act VI in Score Attack
Achieve Max Rank in Onslaught Mode
Land a hit on an enemy's head with a firearm or explosion
Land a leg hit on an enemy with a firearm or explosion
Land a hit on an enemy with an explosion
Finish an entire playthrough of Story Mode in one sitting on Hard difficulty
Finish an entire playthrough of Story Mode in one sitting on Off the Rails difficulty
Unlock Tucker
Unlock Miranda
Unlock Zarina
Unlock Falco Jr.
Unlock Jackie
Unlock Michiko
Unlock Dominic