I liked this one!

import Control.Arrow  
import Control.Monad  
import Data.List  
import Data.Ratio  

simpleTrain = uncurry (%) . (head &&& last) . map read  

compoundTrain input =  
  let a = read $ head input  
      z = read $ last input  
      gs =  
        map  
          ( uncurry (%)  
              . (read *** read . tail)  
              . break (== '|')  
          )  
          $ (tail . init) input  
   in foldl' (/) (a % z) gs  

part1, part2, part3 :: [String] -> Integer  
part1 = floor . (2025 *) . simpleTrain  
part2 = ceiling . (10000000000000 /) . simpleTrain  
part3 = floor . (100 *) . compoundTrain  

main =  
  forM_  
    [ ("everybody_codes_e2025_q04_p1.txt", part1),  
      ("everybody_codes_e2025_q04_p2.txt", part2),  
      ("everybody_codes_e2025_q04_p3.txt", part3)  
    ]  
    $ \(input, solve) -> readFile input >>= print . solve . lines  

Nim

For part 3 I parse gears as tuples, with regular gears having same value on both ends e.g.

3|5 -> (3, 5)
3   -> (3, 3)

proc parseGears(input: string): seq[int] =
  for line in input.splitLines():
    result.add parseInt(line)

proc parseNestedGears(input: string): seq[(int, int)] =
  for line in input.splitLines():
    let nested = line.split('|').mapIt(it.parseInt)
    result.add:
      if nested.len == 1: (nested[0], nested[0])
      else: (nested[0], nested[1])

proc solve_part1*(input: string): Solution =
  let gears = parseGears(input)
  result := 2025 * gears[0] div gears[^1]

proc solve_part2*(input: string): Solution =
  let gears = parseGears(input)
  result := ceil(10000000000000'f64 / (gears[0] / gears[^1])).int

proc solve_part3*(input: string): Solution =
  let gears = parseNestedGears(input)
  let ratios = (0..gears.high-1).mapIt(gears[it][1] / gears[it+1][0])
  result := int(100 * ratios.prod)

Full solution at Codeberg: solution.nim

Rust

use num::{BigInt, Integer};

pub fn solve_part_1(input: &str) -> String {
    let gears: Vec<i64> = input.trim().lines().map(|g| g.parse().unwrap()).collect();
    (2025 * gears[0] / gears.last().unwrap()).to_string()
}

pub fn solve_part_2(input: &str) -> String {
    let gears: Vec<i64> = input.trim().lines().map(|g| g.parse().unwrap()).collect();
    let res = (BigInt::parse_bytes(b"10000000000000", 10).unwrap() * gears.last().unwrap())
        .div_ceil(&(BigInt::ZERO + gears[0]));
    res.to_string()
}

pub fn solve_part_3(input: &str) -> String {
    let mut lines = input.trim().lines();
    let first_gear = BigInt::parse_bytes(lines.next().unwrap().as_bytes(), 10).unwrap();
    let mut nominator: BigInt = first_gear * 100;
    let mut denominator: BigInt = BigInt::ZERO + 1;
    for line in lines {
        let mut split = line.split("|");
        denominator *= BigInt::parse_bytes(split.next().unwrap().as_bytes(), 10).unwrap();
        match split.next() {
            Some(size) => {
                nominator *= BigInt::parse_bytes(size.as_bytes(), 10).unwrap();
            }
            None => {
                break;
            }
        }
    }
    (nominator / denominator).to_string()
}