The Blast Furnace is a 3x3x4 structure made out of Blast Furnace Brick which, like the Coke Oven, is hollow in the middle except on the Blast Furnace two of the blocks are hollow in the middle. It is not suggested to stand in the middle of the hollow portion when placing the last brick, as lava appears in the hollow portion.
The Blast Furnace is used to turn Iron Ingots into Steel Ingots, which are essential in the RailCraft mod. It takes approximately 16 seconds to make a Steel Ingot, meaning timers must be set to four seconds for charcoal, 8 if using Coal Coke, and 16 secs. per Iron Ingot.
Coal (128 EMC) to Charcoal (32 EMC) yields four times as much Charcoal as Coal , but the Blast Furnace requires only twice as much Charcoal as Coal Coke . This way you'll need only half as much Coal to create the same amount of Steel , compared to using Coal Coke created in a Coke Oven. It's also much faster. The loss is that you won't gain the Creosote Oil that's produced when turning the Coal into Coal Coke unless you turn wood into charcoal in a Coke Oven.