In that era, bar cars were common and expected on intercity trains. According to Arthur D. Dubin's Some Classic Trains, the first Tip Top Tap cars appeared in the Hiawatha, when it was placed in service on May 29, 1935 (the author states that the Tip Top Inn was a famed Chicago restaurant atop the Pullman office building in Chicago); the cars were a combined restaurant and bar. The entire train was re-equipped in October 1936, September 1938, and in 1948, and I believe all of them featured a bar car called the Tip Top Tap. A good source would probably be Jim Scribbins's book The Hiawatha Story, but I don't have it. I know I've seen photos of Tip Top Tap cars, and that may be where. By 1940 the Official Guide was showing Tip Top Tap cars (beverages and light meals), in addition to a full diner, on both the Morning and Afternoon Hiawathas, so there apparently was a number of such cars in circulation. No mention of draft beer, however, and surely there would be a reference to that if it had been a feature of the cars.