In the present paper the mechanical behavior of two types of Kunststof Lankhorst Product (KLP) sleeper, namely low-density polyethylene sleeper (LDPE-16) and high-density polyethylene sleeper (HDPE-25) with 16 mm and 25 mm steel bars diameter, are studied. To this end, the static, dynamic, and longtime static three points bending moment tests are performed. The HDPE-25 and LDPE-16 with six strain gauges are mounted on the steel bars to assess their mechanical responses. Moreover, a Finite Element Method (FEM) model is developed to perform sensitivity analysis on 16 mm HDPE (HDPE-16) and 25 mm LDPE (LDPE-25) based on different steel bar diameters. The results showed that the LDPE-16 steel bar yielded under a load of 30 kN for 4 hours, while HDPE-25 showed significant resistance. Numerical results showed that HDPE-25 is overdesigned and can be replaced by LDPE-25, which is lighter in weight and lower in price. The natural frequencies of HDPE-25 were almost 16%, 19%, 16%, and 33% higher than the three first bending frequencies and first torsion frequency of LDPE-25, respectively. These findings prove the better performance of LDPE-25 in the case of preventing resonance. In addition, the exural modulus of HDPE-25 was almost 42%, 45%, and 65% higher than that of HDPE-16, LDPE-25, and LDPE-16, respectively.