Construction project objective achievement measured by whether, or not the projects are completed on time and budget. Most of the road and rail construction projects in Ethiopia exposed to extreme time and cost overruns. It becomes difficult to complete projects on time and budget. To fill gaps, the study was conducted causes of infrastructure project delay and cost escalation in the federal road and rail construction projects. Data gathered in purposive sampling technique from the targeted population of the client, contractor, and consultant of managers and engineers who have experienced delay and cost overruns. In the total of 73 questionnaires were collected, and 18 senior engineers interviewed in mixed research approaches and analyzed data using applied and descriptive statistics by aided in SPSS and RII . To investigate the real extent of time and cost overruns of the road and railway construction projects, secondary data collected from 25 roads and 3 railways projects construction completed from 2014 to May, 2018 and analyzed by using Microsoft Office Excel 2007. From the investigation, it found that 88% of the road and 100% railway construction projects suffered time performance, and 80% of the road and 100% of the railway construction projects also cost overruns from initially expected. Project delay and cost overrun can be minimized only when their causes are indentified. To analyze the major causes of time and cost overrun of the road and railway construction projects, the study classified determinants into time overrun (technical, overconfidence bias and strategic misrepresentation), and cost overrun. The top-ranked five delay factors found incomplete to study before project approval, poor project management & coordination, the right of way issues, inaccurate forecasting of schedule, psychological biases, and political interests. Inflation of material cost, the scope change with the change order, incomplete study project approval, poor bill of quantity & design, and poor project performance monitoring among top ranked cost overrun determinants. The correlation between time overrun variables/or determinants, and time & cost overrun determinants directly affecting the other time and cost overrun variables in the same project. The research findings suggested that the project stakeholder makes sure that the road and rail projects carrying out on time and within estimated budget. Otherwise, it has found time and cost overruns, this damages client and contractor reputation, loss of profit and investment opportunities, inability to deliver the value of money and inefficient use of time, disposing of business activities and create the burden for taxpayers. The study also reminds for decision-makers and practitioners to address the road and railway construction project delay and cost overrun by rewarding well-performed contractors, improve the capacity of staff, make selective public-private-partnership, enhance performance monitoring and information sharing, and use innovative project cost and time forecasting techniques and tools.