Why does Papa Logan have to work on the railroad in Roll of Thunder, Hear My Cry?

Papa has to make extra money to pay for the taxes the Logans owe on their land because the price of cotton dropped due to the Great Depression.

The Logans are farmers.  Even though the Logan family owns land of their own, it is not enough to pay the bills.  The land is valuable, and it is taxed.  The taxes on the land are so high that Papa has to go to find work.  He gets a job working for the railroad to supplement his income.  The crops are not bringing in as much profit as they used to.  This is largely due to the prices the family can get for their crops, which vary from year to year and have been going down.

That was why Papa had gone to work on the railroad.  In 1930 the price of cotton dropped. And so, in the spring of 1931, Papa set out looking for work, going as far north as Memphis and as far south as the Delta country. (Ch. 1)

It is the Great Depression, and everyone is struggling to make ends meet.  Because the price of cotton dropped, working the land is no longer enough to pay the taxes on it and support the family.  If they do not pay the taxes, they will lose the land. 

Because of the need for the railroad job, Papa is often away from home for long stretches at a time.  This leaves the rest of the family alone to have to deal with whatever situation may arise.  Papa is pretty much only there for planting and harvesting the cotton.

Cassie asks her father why he sacrifices so much in order to keep the land.  Her father explains to her the importance of owning land.

“Look out there, Cassie girl. All that belongs to you. You ain’t never had to live on nobody’s place but your own and long as I live and the family survives, you’ll never have to.  That’s important. You may not understand that now, but one day you will. Then you’ll see.” (Ch. 1)

Even though he has to leave his family to make enough money to pay for the land, it is worth it to Papa.  To him, owning land is the thing that is important.  Most of the other families are sharecroppers, which basically means they are renting the land they farm.  To own land is something special, especially when you are an African American family.