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I would have expected a 1(locomotive)-2(carts) to accelerate exactly as fast as a 2-4. Why isn't that so? They should have the same power per weight and so on.

https://wiki.factorio.com/Locomotive#Maximum_speed

train_speed = max(0, abs(train_speed) - train_friction_force ÷ train_weight)
train_speed = train_speed + (10 × number_of_locomotives_in_moving_direction × fuel_acceleration_bonus ÷ train_weight)
train_speed = train_speed × (1 - air_resistance_of_front_rolling_stock ÷ (train_weight ÷ 1000))

After looking at the formulas in the wiki, I am wondering if it might be the air resistance? It looks like both trains will have the same resistance but the bigger train has more weight so the resistance factor will be less.

Amarula wrote: ↑Thu Dec 05, 2019 3:28 pm https://wiki.factorio.com/Locomotive#Maximum_speed

train_speed = max(0, abs(train_speed) - train_friction_force ÷ train_weight)
train_speed = train_speed + (10 × number_of_locomotives_in_moving_direction × fuel_acceleration_bonus ÷ train_weight)
train_speed = train_speed × (1 - air_resistance_of_front_rolling_stock ÷ (train_weight ÷ 1000))

After looking at the formulas in the wiki, I am wondering if it might be the air resistance? It looks like both trains will have the same resistance but the bigger train has more weight so the resistance factor will be less.

That formula governs the maximum speed but not acceleration.

It kind of has to be. I'm just surprised of it being that strong. If you look at that calculator https://calculatorio.com/train_acceleration/ (For 0.16.51 but 0.17 only increased fuel consumption) a 1-2 needs 270 s to reach top speed, a 2-4 needs 13.4s. A 2-10 needs 32 s, 2-19 needs 150.8s but a 2-20 needs 1532.17s.

However I would guess that air_resistance_of_front_rolling_stock should be a constant so a 1-0-0(back facing locomotive) that also needs 13.4s should accelerate twice as fast as a 1-0-1. But the 1-0-1 also needs 271.08 s.