Abstarct: Abstract Optimum use of the rail transportation system requires efforts to develop and improve operating conditions. The purpose of this study is to examine the hardness of the bed and then the "critical speed" of the track as one of the influencing factors on how to operate a railway. For this purpose, the effect of changes in bed hardness and speed on the quality of train movement was investigated in two types of beds, aggregate pavement and concrete slab. So, the dynamic load corresponding to the movement of the train on the rails in the ABAQUS software (using the VDLOAD code written in FORTRAN) is applied to the mentioned bed and the displacement of the railway with three modulus of elasticity of 10, 30 and 60 MPa (with critical speeds of 140, 250 and 350 km/h respectively) under the speeds of 180, 240 and 300 km/h were measured and compared with each other. Also, displacement, speed, and acceleration on sides of the track (at a distance of 2 meters from the path) were evaluated to control the safety conditions. It was observed that the use of concrete slab instead of ballast in the railway track reduces the displacement of the rail by 50% and the displacement around the railway by 75% . The effect of changing the modulus of elasticity of the bed on the vibrations of the ballasted track was 30% and about 16% on the concrete slab track. Also, increasing the speed of the train caused an increase in fluctuations in the concrete bed by 7% and in the ballast bed by 13%. In addition, changing the speed of the train and changing the modulus of elasticity of the bed also had a greater effect on the vibrations around the ballast track than the concrete track. These results showed that the hard bed can have a direct effect on the critical speed of the train. Considering that with the increase in the quality of the subgrade, the critical speed also increases, it is also very important to target the influential characteristics of the subgrades. Considering this issue, the conditions prevailing in the world today have narrowed the field for the use of ballast and factors such as speed, time and increase load شرایط حاکم بر دنیا امروز در مواردی عرصه را برای استفاده از بستر بالاستی تنگ کرده و فاکتورهایی همچون سرعت، زمان و افزایش بار قابل حمل منجر به گرایش پیدا کردن به استفاده از سیستم بستر ریلی پیشرفته و مدرن‌تری همچون دال بتنی سرتاسری به جای به کار بردن تعداد زیادی تراورس بتنی شده است. بنابراین استفاده از این سیستم ریلی برای بهبود بهره‌وری توصیه می‌شود. کلمات کلیدی: بالاست، دال بتنی، مدول الاستیسیته، ارتعاش، اجزای محدود، ABAQUS Abstract Optimum use of the rail transportation system requires efforts to develop and improve operating conditions. The purpose of this study is to examine the hardness of the bed and then the "critical speed" of the track as one of the influencing factors on how to operate a railway. For this purpose, the effect of changes in bed hardness and speed on the quality of train movement was investigated in two types of beds, aggregate pavement and concrete slab. So, the dynamic load corresponding to the movement of the train on the rails in the ABAQUS software (using the VDLOAD code written in FORTRAN) is applied to the mentioned bed and the displacement of the railway with three modulus of elasticity of 10, 30 and 60 MPa (with critical speeds of 140, 250 and 350 km/h respectively) under the speeds of 180, 240 and 300 km/h were measured and compared with each other. Also, displacement, speed, and acceleration on sides of the track (at a distance of 2 meters from the path) were evaluated to control the safety conditions. It was observed that the use of concrete slab instead of ballast in the railway track reduces the displacement of the rail by 50% and the displacement around the railway by 75% . The effect of changing the modulus of elasticity of the bed on the vibrations of the ballasted track was 30% and about 16% on the concrete slab track. Also, increasing the speed of the train caused an increase in fluctuations in the concrete bed by 7% and in the ballast bed by 13%. In addition, changing the speed of the train and changing the modulus of elasticity of the bed also had a greater effect on the vibrations around the ballast track than the concrete track. These results showed that the hard bed can have a direct effect on the critical speed of the train. Considering that with the increase in the quality of the subgrade, the critical speed also increases, it is also very important to target the influential characteristics of the subgrades. Considering this issue, the conditions prevailing in the world today have narrowed the field for the use of ballast and factors such as speed, time and increase load to the tendency to use advanced and more modern rail bed system such as concrete slab instead of using a large number of sleepers. Therefore, it is recommended to use this rail system to improve productivity.