The Ecliptic Express

After the opening cutscene, Rebecca is on her own aboard the stopped train:

• Head west down the train. The second cabin holds three zombies (one west, two east) — drop the western one with a couple of shots and sprint past rather than clearing the room.
• The next cabin is safe. Check both doors along the north wall: one has Handgun Ammo, the other a Green Herb, Ink Ribbons, and the game's first Typewriter. You can leave the Ink Ribbons for now — this is the only save spot available for a while.
• At the western end of the train, examine the dead conductor to find the Train Key. This triggers a cutscene ending with two Cerberus dogs — back up until the camera swings to face them and shoot each one as it leaps for a free knockback.
• Loot Edward's body nearby for more Handgun Ammo, then head back east toward the locked door, dealing with the earlier zombies again along the way.
• Use the Train Key at the door to the east and continue through, taking out the pair of zombies waiting on the other side.

In the dining car, Billy Coen introduces himself — turn him down for now and press on alone. The door east of the dining room is automatic and unpowered; a note on the table explains the switch for it sits on the roof of the train.

• Head upstairs into the back half of the dining car. A new enemy appears — don't fight it, just retreat toward the stairs. Billy intervenes and saves you, and from here on you control both characters together.
• Send all of Rebecca's ammo over to Billy, then take Rebecca to the rear of the train and examine the back window. Climb the ladder to the roof and head south toward the hole in it.
• Flip the power switch near the hole. Leeches immediately drag Rebecca down into the room below and jam the door shut — she needs "something sharp" to get out.
• Loot the room (the Train Key by the door is the priority), then use the service lift inside and send the Train Key down to the first floor.

Control returns to Billy in the dining car, whose only way to reach Rebecca is the service lift.

• Head down through the kitchen behind the automatic door, grab the Train Key from the service lift, then move west back through the train cabins (the same zombies from before are still there).
• At the conductor's office, use the Train Key to get in, then discard it. Grab the Train Map and read the conductor's note about the Briefcase in the closet — leave it for now. Press the switch on the wall to drop a ladder up into the bar.
• In the bar, grab the Ice Pick off the serving tray, and check the small side room for Handgun Ammo (there's also a Hunting Gun, Shotgun Shells, First Aid Spray, and Ink Ribbons here if you want them later).

• Grab the Panel Opener near the front of the bar and head back down the ladder.

• Return east to the dining car kitchen and send the Ice Pick through the service lift to Rebecca. Two burning zombies block the room outside — have Billy handle them (about five rounds each) before Rebecca breaks herself free with the pick.
• Regroup at the conductor's office and drop anything you're not carrying near the save room — go back for the Hunting Gun and the Briefcase while you're at it.
• Use the Panel Opener on the floor panel in the dining kitchen's corner and crawl through with both characters to the rear deck.
• Two Cerberus wait on the other side — knock them down, then loot the destroyed cages for Shotgun Ammo, First Aid Spray, and a Gold Ring. Unlock the nearby door but don't go through yet.

• Take the Hook Shot back to the dining kitchen. A zombie ambushes from the fridge — just dash past it. Two cars west, send Rebecca up a set of stairs and use the Hook Shot with Billy to bring down a ladder at a nearby window.
• Climb onto the train roof and drop through the hole. Clear the zombie inside, grab the second Knife if you want it, and open the Jewelry Box in the closet for the Silver Ring. The Hook Shot has no more use for a long while — drop it here if you need the space.
• Reunite the team and head back to the bar car. Combine the Gold Ring, Silver Ring, and the Briefcase to unlock it, revealing a Blue Keycard. Use it on the electronic door near the conductor's office.

A cutscene reveals the train has been sabotaged to crash. Two brakes need activating at once — one at the rear deck where you found the Hook Shot, one in the front cabin.

• Grab the Magnetic Card in the front cabin, leave Rebecca there, and rush Billy back to the rear deck (dodging the fallen guards outside, who'll get up if you're slow).

• At the rear brake panel, use the Magnetic Card and enter a code whose digits sum to the number shown. If you're not confident doing the math under the timer, use one of these combinations:

  36 = 3×9, 9×1
  67 = 7×9, 4×1
  81 = 8×9, 9×1
  

• Control switches to Rebecca at the front brake panel — same idea, different target number:

  42 = 4×9, 6×1
  53 = 5×9, 8×1
  67 = 7×9, 4×1
  

• The train crashes to a halt. Three zombies close in immediately — deal with them and grab anything scattered nearby (items you left behind elsewhere on the train end up here too). Head through the southern door, drop into the sewage below, and climb the ladder up into the Umbrella Training Facility.