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Regarding drivers underneath the firebox on modern locos, the type of coal used also affects what type of firebox can be used. UP fireboxes are long with lots of grate area, but relatively shallow, so it's possible to fit reasonably large drivers underneath. The Berkshire is usually talked about as being first, but the UP 4-12-2's had a large firebox with the last pair of 67" drivers beneath it. Challengers with 69" drivers were similar, while Big Boys had 2 pairs of 68" drivers beneath the firebox. The Allegheny and some other eastern locos needed much deeper fireboxes for the hotter burning coal they used, so the entire firebox had to be supported by a trailing truck.
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