When placing more tnt minecrafts on rail at once, they will doesnt explode. They will glitch and stop. The tnt in them will play explosion animation. Then the game will crash with OutOfMemory exception.

— BEGIN ERROR REPORT 39215b3 --------
Full report at:
C:\Users--------\AppData\Roaming\.minecraft\crash-reports\crash-2013-01-10_20.46.04-server.txt
Please show that file to Mojang, NOT just this screen!

Generated 10.1.2013 20:46

– Head –
Stacktrace:
at java.util.HashMap.resize(Unknown Source)
at java.util.HashMap.addEntry(Unknown Source)
at java.util.HashMap.put(Unknown Source)
at java.util.HashSet.add(Unknown Source)
at yi.a(SourceFile:74)
at in.a(SourceFile:659)
at yn.a(SourceFile:1518)
at qi.c(SourceFile:63)
at qi.l_(SourceFile:45)
at yn.a(SourceFile:1312)
at in.a(SourceFile:480)
at yn.g(SourceFile:1290)

– Entity being ticked –
Details:
Entity Type: null (qi)
Entity ID: 38022
Name: unknown
Exact location: 186,50, 63,35, 118,70
Block location: World: (186,63,118), Chunk: (at 10,3,6 in 11,7; contains blocks 176,0,112 to 191,255,127), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Momentum: -0,00, 0,00, 35261,36
Stacktrace:
at yn.h(SourceFile:1197)
at in.h(SourceFile:374)

– Affected level –
Details:
Level name: New World
All players: 1 total; [iq['dobrakmato'/12832, l='New World', x=175,79, y=71,00, z=108,61]]
Chunk stats: ServerChunkCache: 4889 Drop: 0
Level seed: -1078509310
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (144,64,256), Chunk: (at 0,4,0 in 9,16; contains blocks 144,0,256 to 159,255,271), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 15597 game time, 1322 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 8724 (now: false), thunder time: 90361 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
at net.minecraft.server.MinecraftServer.r(SourceFile:744)
at net.minecraft.server.MinecraftServer.q(SourceFile:679)
at bfd.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:613)
at fy.run(SourceFile:849)

– System Details –
Details:
Minecraft Version: 13w02a
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_10, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 98762952 bytes (94 MB) / 909049856 bytes (866 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 3995 (223720 bytes; 0 MB) allocated, 3722 (208432 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
Profiler Position: N/A (disabled)
Vec3 Pool Size: 920 (51520 bytes; 0 MB) allocated, 857 (47992 bytes; 0 MB) used
Player Count: 1 / 8; [iq['dobrakmato'/12832, l='New World', x=175,79, y=71,00, z=108,61]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

java.lang.OutOfMemoryError: Java heap space
at java.util.HashMap.resize(Unknown Source)
at java.util.HashMap.addEntry(Unknown Source)
at java.util.HashMap.put(Unknown Source)
at java.util.HashSet.add(Unknown Source)
at yi.a(SourceFile:74)
at in.a(SourceFile:659)
at yn.a(SourceFile:1518)
at qi.c(SourceFile:63)
at qi.l_(SourceFile:45)
at yn.a(SourceFile:1312)
at in.a(SourceFile:480)
at yn.g(SourceFile:1290)
at yn.h(SourceFile:1197)
at in.h(SourceFile:374)
at net.minecraft.server.MinecraftServer.r(SourceFile:744)
at net.minecraft.server.MinecraftServer.q(SourceFile:679)
at bfd.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:613)
at fy.run(SourceFile:849)
— END ERROR REPORT 47e4190 ----------

Also I hope the classes names (randomchars) and etc.. will help you. The coe is a way more clear