If you are a fresh engineering graduate, its unlikely that your CV will be considered through referrals as Alstom India primarily hires freshers through Campus.
For engineers with experience, the best way to ensure that your profile is screened is through Placement Consultants or referrals of employees working with Alstom. Typically, there is a 1st round of technical interview (telephonic or face to face). Clearing this round will take you to the next level of interview which will be done by the technical / functional / department head. Provided you clear both, you will then need to face a HR (in depth) interview.
The turn around time from the first contact till the offer may vary in the type of position. For team member requirements (2-6 yrs experience), it may be fast and 15 days should be what you can expect on an average. For senior positions, it may go up to 30 days since they will normally need to go through additional rounds of interview.
For support functions (HR, Quality, Sourcing, Finance, etc) - there is no specific turn around time. It may be quick or can be delayed depending on the position and te urgency of the need.
For me, the time it took for decision making was apt! 2 levels of Interview and I had the verdict!