I think two possible themes are the themes of bravery and heroism. Baldeo has a seemingly easy job. He must make sure a train tunnel is free of obstructions and light a lamp that signals that fact to the train. Baldeo has heard of reports of a dangerous tiger in the area (in addition to other dangerous jungle animals), but he bravely and confidently does his job. To his son, Tembo, Baldeo is a heroic figure that stands bravely against the jungle animals. Baldeo portrays that figure to the reader as well. When the tiger shows up and attacks, Baldeo doesn't panic and run. Instead, he stands and fights. In fact, the reader is even given hope that Baldeo will be victorious over the tiger.
Baldeo, however, was ready. With a marvelously agile leap he avoided the paw and brought his axe down on the animal’s shoulder. The tiger gave a roar and attempted to close in. Again Baldeo drove his axe which caught the tiger on the shoulder, almost severing the leg.
Unfortunately, the tiger is victorious over Baldeo. The themes of man's mortality and the danger of nature is shown in that fact. Baldeo may be confident in his skills and his axe, but ultimately Mother Nature proves to be the more dangerous of the two. Young Tembo learns a hard lesson that day about nature and man's mortality. Man's mortality isn't the only theme of the story though. The tiger is killed by the oncoming train, which shows that nature is just as mortal as man.
I hate to sound like a Lion King cliche, but another theme of the story might be "the circle of life." Baldeo is master of himself, his family, and the tunnel. He lives and then dies. The tiger, for a moment, is master of that area and the tunnel. Then the tiger is killed by the train, and Tembo takes his father's previous position as head of the family and tunnel guardian. The circle is made complete.