The purple Counthash in west Fort Law must be entered. It's under the elevated railroad track west from a small park with four telephones. The objective is to stop a train from moving Sonetti's drugs around the city. The player must take the briefcase of explosives they are given upon entering the Counthash to the station in west Schlechberg, although the dialogue says southeast Guernsey City. The train must be entered and exited to plant the explosives. The mission is completed after the explosion.
The player is awarded 50,000 points (50,000 x Score Multiplier) and a +1 increase of score multiplier for successfully completing the mission.
This mission is not included in the console version. Due to graphical limitations, trains do not appear. The player will still receiver a pager message regarding the Counthash's location, however, it will not be there.
It can happen that the train does not arrive at the Schlechberg station in time, even if the protagonist gets there immediately after taking the mission. In this case, the mission is failed. (It happens randomly, even if the player had not hijacked, nor crashed any trains.)
Grand Theft Auto 1 PC Liberty City Chapter 1 - Other Vehicle Mission 6