Asynchronous automated production lines with finite buffer capacity producing discrete parts are adopted in the production of most of the mid to high volume products and are therefore very common in real plants. Existing ana- lytical method to model such kind of systems are based on simplifying assumptions which however are becoming less and less strong. The paper proposes an analysis of the rationale behind these assumptions and how they affect performance estimation in the case of tandem production lines. A new model based on a continuous representa- tion of the material flow coupled with a threshold based control policy is proposed. The model introduces milder assumptions in comparison with existing models and allows to obtain precise evaluation of the performance of asynchronous automated production lines.