I was surprised recently as I visited the Wutachtal railway in southern Germany. A particular section of this railway makes extremes detours with multiple bridges and tunnels in order to climb 231m while having a railway steep of only 1%.

According to the museum's explaination, the steep of 1% was required because they wanted to allow heavy military freight trains to circulate on this railway. However, steeper railways are very common, including rails which allows heavy freight trains. For instance, according to German language Wikipedia:

• The Gotthard railway has a max steepness of 2.8%
• The Simplon railway as well as the Neuchâtel–Pontarlier railway a max steepness of 2.5%
• The Arlberg railway in Austria has a steepness of 3%.
• The line between Vevey and Chexbres has a steepness of 4% and light freight trains circulates on this line.

Some trains which only allows passenger train have much steepers rails. The Uetli train in Zürich has a steepness of almost 8%.

Since the maximum allowed steepness is a major factor when designing a train line, I really wonder what factors determined it. Is it the train's weight or Locomotive's technology ? Why could Switzerland and Austria allows freight trains on 3% or even 4% steep tracks while Germany needed to limit itself 1% ?