Crossing the Trunk Line, Crossing the Sluice Gate, and To the Sector 7 Slums are quests in Chapter 10, "Rough Waters", in Final Fantasy VII Remake, taking place in the Sewer System. After Cloud Strife, Aerith Gainsborough, and Tifa Lockhart fall into the Sewer System, Tifa notes that the group have reached the trunk line, point where Sector 6 ends and Sector 7 begins. The three then follow the path around the line to get to Sector 7.
After entering the trunk line, head right to a wider platform, where a blugu and some wererats appear. These are simple enemies that can be defeated with normal attacks or stronger character abilities, though blugu's bubbles should be avoided as they inflict Sleep. After this, descend a ladder down to a lower area, where a sluice switch can be used to open a floodgate allowing the canal to be traversed. Here, blugus and a scissorclaw can be fought. The scissorclaw is best defeated with magic spells, particular those from the Ice Materia, as it resists physical attacks, though strong character abilities and Tifa's Unbridled Strength martial techniques are also sufficient. Blugus take higher priority to prevent them from sleeping characters.
Along the lower path, go under the gates to proceed. Defeat the scissorclaws and blugu using the same tactics as before, and then head right up the ladder to an upper ledge and pull the lever to lower the sluice gate, then head up another ladder to the right of the lever. A Poison Materia can be found by proceeding backwards, while heading forward will allow the sluice gate (if it was lowered) to be walked across to another side. Descend the ladder to the right, and down another ladder onto the canal, where a sahagin is fought. This enemy can be defeated quickly with Fire Materia spells, or with character abilities. Head right up the ladder and use a lever to flood the canal, creating a bridge to cross to the other side. When beginning to cross, a cutscene commences.
After the cutscene ends, the party crosses the bridge. Follow Aerith and Tifa up the ladder then approach them, leading to another cutscene. Follow Tifa and Aerith through a series of hallways, leading to a room with a vending machine and a bench. Use the bench to heal, and restock on any items, then proceed forward along a corridor and down a ladder, leading to Aqueduct 2. There is a fork in the road from here; heading straight (south on the map) leads to a room with a treasure chest for two orbs of gravity , while heading right (west on the map) progresses forward. Taking the ladder down at the end of the path leads to a room with a ladder on the right (west side on the map) that leads upward to Aqueduct 1, a series of interconnected rooms.
Here is a series of five interconnected rooms. After starting in the eastern room, head to the northeast room, where a blugu, a scissorclaw, and a wererat are fought. After doing this, a Warding Materia orb can be seen on the other side; this unlocks the discovery sidequest "On the Other Side" to obtain it. This materia can reduce the duration of debuffs, making it very useful. Aside from this, in the lower rooms, a pair of revival earrings can be obtained by reaching the southeastern-most room and heading north to the bars; along the way, enemies of the same type are fought again, and the same tactics apply. To proceed with the quest, from the northwest room, simply head west again into a small room, and then follow the linear path from there into a corridor with a wide, open room. This leads to the next quest, "To the Surface".
Hard mode tips
As with the previous quest, the important thing is to preserve MP. Use Prayer Materia or Chakra Materia if healing is essential, and using weapons with Bloodsucker may be ideal. Aerith can use Soul Drain offensively, and smashing crates for mako shards to restore MP is ideal. Beyond this, the quest plays similarly to before.