Yes I think 6.3 cm is safe. . U ca travel once chk with Ur doctor. And it's not much low lying and will be corrected in later mmonths...

Sorry, didn't read the words properly... I was told by doctor about Bus travel, You can once check with doctor about train travel ...

Please don't travel when you have low lying placenta.... Even I had the same problem at 15 weeks and doctor asked me to be very careful... I didn't travel anywhere till my placenta was in normal position and that's why I was able to do normal delivery of baby now

6.3 cm is quite far away make sure it is cm and not mm.
Travel by train should be safe

Yes dear. ...if possible please avoid travelling. .n tk proper rest.