From a Web site devoted to the Altoona Works (past and present-- there are also drawings of the cabs used on current NS diesel rebuilds), here is the I1 as built: note the three blind driving axles:
http://www.altoonaworks.info/graphics/drawing_i1s.jpg" onclick="window.open(this.href);return false;
(From the format and, particularly, the style of the caption, I'm guessing that this drawing appeared in some edition of the "Locomotive Cyclopedia.")
The PRR's penchant for putting flangeless drivers on their locomotives wasn't limited to types with five driving axles-- here is the H6 Consolidation, with the second and third drivers blind:
http://www.altoonaworks.info/graphics/drawing_h6.jpg" onclick="window.open(this.href);return false;
Nor was the use of blind drivers on eight-coupled power limited to the very start of the 20th C-- here is the C1 switcher of 1925:
http://www.altoonaworks.info/graphics/drawing_c1.jpg" onclick="window.open(this.href);return false;
(Mind you, if Alvin Stauffer's "Pennsy Power" is to be trusted, the C1 wasn't one of the PRR's outstanding successes, being derailment prone…)
Indeed, sometimes the centre drivers of six-coupled locomotives were flangeless-- here is a well-known Pacific:
http://www.altoonaworks.info/graphics/drawing_k4.jpg" onclick="window.open(this.href);return false;
(Another drawing shows the experimental K5 Pacific of 1929… also with the centre driver blind.)
Indeed… Here is an 80-inch drivered Atlantic, apparently built for Atlantic City trains (where the Reading even used 4-2-2 power):
http://www.altoonaworks.info/graphics/drawing_e1.jpg" onclick="window.open(this.href);return false;
It LOOKS as if the first driver was blind-- can this really be so?
(Cross posted to Steam Locomotives forum.)