Secret Passageway, Stairway Detour, and The Road to Mako Reactor 5 are quests in Chapter 5, "Dogged Pursuit", in Final Fantasy VII Remake. While Cloud Strife, Barret Wallace, and Tifa Lockhart travel through the Corkscrew Tunnel to find Mako Reactor 5, the two find the train yard gate locked. Barret suggests a secret passageway behind some stairs adjacent to the gate.
Walkthrough[]
Follow Barret to the stairway, and head up to a door. Open it and follow the linear path through walkways up to a staircase. Head up the staircase, and use the door at the end leads to a room with a vending machine, where items and the "Stamp" song can be bought, and a 
Lightning Materia can be found on the floor by the lockers. Continue through a staircase up to a break room, where an elite security officer is encountered. They are weak to 
Fire, but character abilities should effectively dispatch them. The door at the end leads out to a ladder down into a more open room. Graffiti of stamp is found at the south end of the room, and a boss battle begins against Crab Warden.
Crab Warden is a very durable enemy, which attacks with machine guns and also by stomping its legs. It is weak to 
Lightning, making 
Thunder effective against it. Summon Materia can also be used in this battle, meaning Ifrit can be summoned. In the first phase of the boss, the party must target its legs. The machine will focus on one character, against which a red target reticule will appear. This character should then keep a distance from the machine and guard against the attack, though the player can simply change character to focus on its legs. To damage it, keep Barret at a distance from the machine shooting at a leg, while Tifa and Cloud attack its legs once they are stationary, either with Unbridled Strength martial techniques or Cloud's Punisher Mode respectively, to destroy one. Doing so will pressure the machine, meaning characters should lay into it to stagger it. Tifa should use an Unbridled Strength martial technique to increase the damage it receives and target the generator.
After it sustains damage, it enters the next phase of battle, in which it will summon Slug-Rays occasionally, which can be dealt with by Barret with ranged attacks as Cloud and Tifa focus on the legs. Otherwise, the battle is the same as before; cripple its legs, but make sure to keep a distance when it uses close-ranged attacks. In the third phase of battle, the legs are no longer vulnerable, it can fire a missile barrage at a range, and will occasionally cause a lightning surge that will be deadly to characters standing on the tracks. If this happens, switch to Barret to still attack through this. It can also focus down one character with attacks, at which point, the best option is for the player to switch to another character to continue dealing damage. Dodge the missiles and heal often, and focus on its right and left auxiliary weaponry. When the pilot is exposed, focus down the pilot, and otherwise continue the same strategy. Once the pilot is crippled, the weak spot is exposed, for Cloud and Tifa to attack it, as the boss is destroyed.
After the boss is down, a cutscene commences. After this, Follow Barret and Tifa through the small gap between two large containers, then head left to a small room and use the ladder to the Service Tunnel. Once down here, activate a switch, then use another to activate an elevator down to Mako Reactor 5. The quest then continues in "Head for Section G" in the next chapter, "Light the Way".
Hard mode tips[]
Crab Warden is more durable and more deadly than before, but there are more ways to deal with it. Barret should make use of Lifesaver often, and can also make use of Barrier Materia to provide resistance to its attacks; if paired with 
Magnify Materia, this can be applied to all party members. Time Materia can be used to good effect to cast Haste and buff party members. Tifa's Focused Strike and Starshower allow her to damage the Warden while being mostly invulnerable herself, and Cloud's MP pool allows him to cast Thundaga often in battle, especially if paired with MP Absorption Materia.
Defeating Crab Warden gives Barret the Sharpshooter's Companion Vol. V.