Gas separation such as $ CO_2 $ from $ N_2 $ in flue gas is an important step in reducing greenhouse gas emission. We discuss a separation method that moves mixed gas between two tracks at different pressures with ever-increasing purity at each stage along the tracks. Designs for adsorbents driven by volumetric pumps and membranes driven by a pressure drop are illustrated. For selectivity exceeding 10 in a three-stage system, the purity of the $ CO_2 $ output is 97.8% with 99.75% recovered. For a six-stage example, the purity of $ CO_2 $ is 99.996% and 99.9996% is recovered. The cost of compression is estimated from the number of times the two components have to be pressurized as they move between the tracks. For large selectivity, the effective number of times for $ CO_2 $ is 2 for the adsorbent case and 3 for the membranes in the three-stage design; these numbers are 5 and 6 in the six-stage design. As a result, the compressional energy requirement per input mole is $ RT ln (C)(1 + Af )$ in the isothermal case for compression factor C, effective number of re-compressions A, and input $ CO_2 $ fraction $ f $. This compressional energy exceeds the minimum energy from entropy by the factor ~ $ (1 + Af )/f $ if $ C = 1/f $ for efficient membrane use.