The current paper aims to investigate stability of side’s slops of geocell-reinforced railway embankments. For this purpose, firstly a set of six 1:20 scaled models including a reference embankment and geocell-reinforced embankments was constructed in a loading chamber and their load-settlement behaviour was assessed. In the next stage, 3D FEM models of the embankments were developed and the relevant results were verified against the laboratory test outcomes. In continue, on the basis of verified models, the scaled up railway embankments were simulated and the real train loading applied to the models. In this matter, a wide-ranging parametric study was performed on the embankment soil properties ranged from poor (ST1) to high strength (ST5) materials, geocell elasticity modulus (E), number of geocell layers (N) and their vertical location in the embankment body (U) to achieve a minimum embankment sliding safety factor (SF) of 1.5. Outcomes indicate that geocell opening size, stiffness and the placement position play an important role where the concern is to stabilize the embankments' sides slopes. It was found that middle of the embankment was the best position of geocell layers. Elasticity modulus of 1400 MPa and opening size of 245*210 mm were also determined as the optimum for geocell layers.