Extended rods

[Overmod]

I believe you're mentioning what are sometimes called 'tailrods' -- extension of the piston rod forward as well as backward to the crosshead. Primary 'reason' was to provide support for the piston at front as well as rear (so that no provision for supporting the weight of the piston inside the cylinder had to be made, and the ring sealing could be a bit less involved). A secondary reason was that the steam volumes on each side of the piston were equivalent, theoretically giving a bit better balance and smoother running. In US practice, tailrods were generally found unnecessary and their increased maintenance cost undesirable, although you do see them in some overseas applications and on some modern designs (cf. the ACE3000)

[rogruth]

What was the reason for extended piston rods used on some locomotives in the early 20th century? I remember seeing them on some PRR engines and in photos.